Quelle direct-io.c Sprache: C

// SPDX-License-Identifier: GPL-2.0-only
/*
* fs/direct-io.c
*
* Copyright (C) 2002, Linus Torvalds.
*
* O_DIRECT
*
* 04Jul2002 Andrew Morton
* Initial version
* 11Sep2002 janetinc@us.ibm.com
* added readv/writev support.
* 29Oct2002 Andrew Morton
* rewrote bio_add_page() support.
* 30Oct2002 pbadari@us.ibm.com
* added support for non-aligned IO.
* 06Nov2002 pbadari@us.ibm.com
* added asynchronous IO support.
* 21Jul2003 nathans@sgi.com
* added IO completion notifier.
*/

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/types.h>
#include <linux/fs.h>
#include <linux/mm.h>
#include <linux/slab.h>
#include <linux/highmem.h>
#include <linux/pagemap.h>
#include <linux/task_io_accounting_ops.h>
#include <linux/bio.h>
#include <linux/wait.h>
#include <linux/err.h>
#include <linux/blkdev.h>
#include <linux/buffer_head.h>
#include <linux/rwsem.h>
#include <linux/uio.h>
#include <linux/atomic.h>

#include "internal.h"

/*
* How many user pages to map in one call to iov_iter_extract_pages().  This
* determines the size of a structure in the slab cache
*/
#define DIO_PAGES 64

/*
* Flags for dio_complete()
*/
#define DIO_COMPLETE_ASYNC  0x01 /* This is async IO */
#define DIO_COMPLETE_INVALIDATE  0x02 /* Can invalidate pages */

/*
* This code generally works in units of "dio_blocks".  A dio_block is
* somewhere between the hard sector size and the filesystem block size.  it
* is determined on a per-invocation basis.   When talking to the filesystem
* we need to convert dio_blocks to fs_blocks by scaling the dio_block quantity
* down by dio->blkfactor.  Similarly, fs-blocksize quantities are converted
* to bio_block quantities by shifting left by blkfactor.
*
* If blkfactor is zero then the user's request was aligned to the filesystem's
* blocksize.
*/

/* dio_state only used in the submission path */

struct dio_submit {
struct bio *bio;  /* bio under assembly */
unsigned blkbits;  /* doesn't change */
unsigned blkfactor;  /* When we're using an alignment which
   is finer than the filesystem's soft
   blocksize, this specifies how much
   finer.  blkfactor=2 means 1/4-block
   alignment.  Does not change */
unsigned start_zero_done; /* flag: sub-blocksize zeroing has
   been performed at the start of a
   write */
int pages_in_io;  /* approximate total IO pages */
sector_t block_in_file;  /* Current offset into the underlying
   file in dio_block units. */
unsigned blocks_available; /* At block_in_file.  changes */
int reap_counter;  /* rate limit reaping */
sector_t final_block_in_request;/* doesn't change */
int boundary;   /* prev block is at a boundary */
get_block_t *get_block;  /* block mapping function */

loff_t logical_offset_in_bio; /* current first logical block in bio */
sector_t final_block_in_bio; /* current final block in bio + 1 */
sector_t next_block_for_io; /* next block to be put under IO,
   in dio_blocks units */

/*
* Deferred addition of a page to the dio.  These variables are
* private to dio_send_cur_page(), submit_page_section() and
* dio_bio_add_page().
*/
struct page *cur_page;  /* The page */
unsigned cur_page_offset; /* Offset into it, in bytes */
unsigned cur_page_len;  /* Nr of bytes at cur_page_offset */
sector_t cur_page_block; /* Where it starts */
loff_t cur_page_fs_offset; /* Offset in file */

struct iov_iter *iter;
/*
* Page queue.  These variables belong to dio_refill_pages() and
* dio_get_page().
*/
unsigned head;   /* next page to process */
unsigned tail;   /* last valid page + 1 */
size_t from, to;
};

/* dio_state communicated between submission path and end_io */
struct dio {
int flags;   /* doesn't change */
blk_opf_t opf;   /* request operation type and flags */
struct gendisk *bio_disk;
struct inode *inode;
loff_t i_size;   /* i_size when submitted */
dio_iodone_t *end_io;  /* IO completion function */
bool is_pinned;   /* T if we have pins on the pages */

void *private;   /* copy from map_bh.b_private */

/* BIO completion state */
spinlock_t bio_lock;  /* protects BIO fields below */
int page_errors;  /* err from iov_iter_extract_pages() */
int is_async;   /* is IO async ? */
bool defer_completion;  /* defer AIO completion to workqueue? */
bool should_dirty;  /* if pages should be dirtied */
int io_error;   /* IO error in completion path */
unsigned long refcount;  /* direct_io_worker() and bios */
struct bio *bio_list;  /* singly linked via bi_private */
struct task_struct *waiter; /* waiting task (NULL if none) */

/* AIO related stuff */
struct kiocb *iocb;  /* kiocb */
ssize_t result;                 /* IO result */

/*
* pages[] (and any fields placed after it) are not zeroed out at
* allocation time.  Don't add new fields after pages[] unless you
* wish that they not be zeroed.
*/
union {
  struct page *pages[DIO_PAGES]; /* page buffer */
  struct work_struct complete_work;/* deferred AIO completion */
};
} ____cacheline_aligned_in_smp;

static struct kmem_cache *dio_cache __ro_after_init;

/*
* How many pages are in the queue?
*/
static inline unsigned dio_pages_present(struct dio_submit *sdio)
{
return sdio->tail - sdio->head;
}

/*
* Go grab and pin some userspace pages.   Typically we'll get 64 at a time.
*/
static inline int dio_refill_pages(struct dio *dio, struct dio_submit *sdio)
{
struct page **pages = dio->pages;
const enum req_op dio_op = dio->opf & REQ_OP_MASK;
ssize_t ret;

ret = iov_iter_extract_pages(sdio->iter, &pages, LONG_MAX,
         DIO_PAGES, 0, &sdio->from);

if (ret < 0 && sdio->blocks_available && dio_op == REQ_OP_WRITE) {
  /*
* A memory fault, but the filesystem has some outstanding
* mapped blocks.  We need to use those blocks up to avoid
* leaking stale data in the file.
*/
  if (dio->page_errors == 0)
   dio->page_errors = ret;
  dio->pages[0] = ZERO_PAGE(0);
  sdio->head = 0;
  sdio->tail = 1;
  sdio->from = 0;
  sdio->to = PAGE_SIZE;
  return 0;
}

if (ret >= 0) {
  ret += sdio->from;
  sdio->head = 0;
  sdio->tail = (ret + PAGE_SIZE - 1) / PAGE_SIZE;
  sdio->to = ((ret - 1) & (PAGE_SIZE - 1)) + 1;
  return 0;
}
return ret;
}

/*
* Get another userspace page.  Returns an ERR_PTR on error.  Pages are
* buffered inside the dio so that we can call iov_iter_extract_pages()
* against a decent number of pages, less frequently.  To provide nicer use of
* the L1 cache.
*/
static inline struct page *dio_get_page(struct dio *dio,
     struct dio_submit *sdio)
{
if (dio_pages_present(sdio) == 0) {
  int ret;

  ret = dio_refill_pages(dio, sdio);
  if (ret)
   return ERR_PTR(ret);
  BUG_ON(dio_pages_present(sdio) == 0);
}
return dio->pages[sdio->head];
}

static void dio_pin_page(struct dio *dio, struct page *page)
{
if (dio->is_pinned)
  folio_add_pin(page_folio(page));
}

static void dio_unpin_page(struct dio *dio, struct page *page)
{
if (dio->is_pinned)
  unpin_user_page(page);
}

/*
* dio_complete() - called when all DIO BIO I/O has been completed
*
* This drops i_dio_count, lets interested parties know that a DIO operation
* has completed, and calculates the resulting return code for the operation.
*
* It lets the filesystem know if it registered an interest earlier via
* get_block.  Pass the private field of the map buffer_head so that
* filesystems can use it to hold additional state between get_block calls and
* dio_complete.
*/
static ssize_t dio_complete(struct dio *dio, ssize_t ret, unsigned int flags)
{
const enum req_op dio_op = dio->opf & REQ_OP_MASK;
loff_t offset = dio->iocb->ki_pos;
ssize_t transferred = 0;
int err;

/*
* AIO submission can race with bio completion to get here while
* expecting to have the last io completed by bio completion.
* In that case -EIOCBQUEUED is in fact not an error we want
* to preserve through this call.
*/
if (ret == -EIOCBQUEUED)
  ret = 0;

if (dio->result) {
  transferred = dio->result;

  /* Check for short read case */
  if (dio_op == REQ_OP_READ &&
      ((offset + transferred) > dio->i_size))
   transferred = dio->i_size - offset;
  /* ignore EFAULT if some IO has been done */
  if (unlikely(ret == -EFAULT) && transferred)
   ret = 0;
}

if (ret == 0)
  ret = dio->page_errors;
if (ret == 0)
  ret = dio->io_error;
if (ret == 0)
  ret = transferred;

if (dio->end_io) {
  // XXX: ki_pos??
  err = dio->end_io(dio->iocb, offset, ret, dio->private);
  if (err)
   ret = err;
}

/*
* Try again to invalidate clean pages which might have been cached by
* non-direct readahead, or faulted in by get_user_pages() if the source
* of the write was an mmap'ed region of the file we're writing.  Either
* one is a pretty crazy thing to do, so we don't support it 100%.  If
* this invalidation fails, tough, the write still worked...
*
* And this page cache invalidation has to be after dio->end_io(), as
* some filesystems convert unwritten extents to real allocations in
* end_io() when necessary, otherwise a racing buffer read would cache
* zeros from unwritten extents.
*/
if (flags & DIO_COMPLETE_INVALIDATE &&
     ret > 0 && dio_op == REQ_OP_WRITE)
  kiocb_invalidate_post_direct_write(dio->iocb, ret);

inode_dio_end(dio->inode);

if (flags & DIO_COMPLETE_ASYNC) {
  /*
* generic_write_sync expects ki_pos to have been updated
* already, but the submission path only does this for
* synchronous I/O.
*/
  dio->iocb->ki_pos += transferred;

  if (ret > 0 && dio_op == REQ_OP_WRITE)
   ret = generic_write_sync(dio->iocb, ret);
  dio->iocb->ki_complete(dio->iocb, ret);
}

kmem_cache_free(dio_cache, dio);
return ret;
}

static void dio_aio_complete_work(struct work_struct *work)
{
struct dio *dio = container_of(work, struct dio, complete_work);

dio_complete(dio, 0, DIO_COMPLETE_ASYNC | DIO_COMPLETE_INVALIDATE);
}

static blk_status_t dio_bio_complete(struct dio *dio, struct bio *bio);

/*
* Asynchronous IO callback.
*/
static void dio_bio_end_aio(struct bio *bio)
{
struct dio *dio = bio->bi_private;
const enum req_op dio_op = dio->opf & REQ_OP_MASK;
unsigned long remaining;
unsigned long flags;
bool defer_completion = false;

/* cleanup the bio */
dio_bio_complete(dio, bio);

spin_lock_irqsave(&dio->bio_lock, flags);
remaining = --dio->refcount;
if (remaining == 1 && dio->waiter)
  wake_up_process(dio->waiter);
spin_unlock_irqrestore(&dio->bio_lock, flags);

if (remaining == 0) {
  /*
* Defer completion when defer_completion is set or
* when the inode has pages mapped and this is AIO write.
* We need to invalidate those pages because there is a
* chance they contain stale data in the case buffered IO
* went in between AIO submission and completion into the
* same region.
*/
  if (dio->result)
   defer_completion = dio->defer_completion ||
        (dio_op == REQ_OP_WRITE &&
         dio->inode->i_mapping->nrpages);
  if (defer_completion) {
   INIT_WORK(&dio->complete_work, dio_aio_complete_work);
   queue_work(dio->inode->i_sb->s_dio_done_wq,
       &dio->complete_work);
  } else {
   dio_complete(dio, 0, DIO_COMPLETE_ASYNC);
  }
}
}

/*
* The BIO completion handler simply queues the BIO up for the process-context
* handler.
*
* During I/O bi_private points at the dio.  After I/O, bi_private is used to
* implement a singly-linked list of completed BIOs, at dio->bio_list.
*/
static void dio_bio_end_io(struct bio *bio)
{
struct dio *dio = bio->bi_private;
unsigned long flags;

spin_lock_irqsave(&dio->bio_lock, flags);
bio->bi_private = dio->bio_list;
dio->bio_list = bio;
if (--dio->refcount == 1 && dio->waiter)
  wake_up_process(dio->waiter);
spin_unlock_irqrestore(&dio->bio_lock, flags);
}

static inline void
dio_bio_alloc(struct dio *dio, struct dio_submit *sdio,
       struct block_device *bdev,
       sector_t first_sector, int nr_vecs)
{
struct bio *bio;

/*
* bio_alloc() is guaranteed to return a bio when allowed to sleep and
* we request a valid number of vectors.
*/
bio = bio_alloc(bdev, nr_vecs, dio->opf, GFP_KERNEL);
bio->bi_iter.bi_sector = first_sector;
if (dio->is_async)
  bio->bi_end_io = dio_bio_end_aio;
else
  bio->bi_end_io = dio_bio_end_io;
if (dio->is_pinned)
  bio_set_flag(bio, BIO_PAGE_PINNED);
bio->bi_write_hint = file_inode(dio->iocb->ki_filp)->i_write_hint;

sdio->bio = bio;
sdio->logical_offset_in_bio = sdio->cur_page_fs_offset;
}

/*
* In the AIO read case we speculatively dirty the pages before starting IO.
* During IO completion, any of these pages which happen to have been written
* back will be redirtied by bio_check_pages_dirty().
*
* bios hold a dio reference between submit_bio and ->end_io.
*/
static inline void dio_bio_submit(struct dio *dio, struct dio_submit *sdio)
{
const enum req_op dio_op = dio->opf & REQ_OP_MASK;
struct bio *bio = sdio->bio;
unsigned long flags;

bio->bi_private = dio;

spin_lock_irqsave(&dio->bio_lock, flags);
dio->refcount++;
spin_unlock_irqrestore(&dio->bio_lock, flags);

if (dio->is_async && dio_op == REQ_OP_READ && dio->should_dirty)
  bio_set_pages_dirty(bio);

dio->bio_disk = bio->bi_bdev->bd_disk;

submit_bio(bio);

sdio->bio = NULL;
sdio->boundary = 0;
sdio->logical_offset_in_bio = 0;
}

/*
* Release any resources in case of a failure
*/
static inline void dio_cleanup(struct dio *dio, struct dio_submit *sdio)
{
if (dio->is_pinned)
  unpin_user_pages(dio->pages + sdio->head,
     sdio->tail - sdio->head);
sdio->head = sdio->tail;
}

/*
* Wait for the next BIO to complete.  Remove it and return it.  NULL is
* returned once all BIOs have been completed.  This must only be called once
* all bios have been issued so that dio->refcount can only decrease.  This
* requires that the caller hold a reference on the dio.
*/
static struct bio *dio_await_one(struct dio *dio)
{
unsigned long flags;
struct bio *bio = NULL;

spin_lock_irqsave(&dio->bio_lock, flags);

/*
* Wait as long as the list is empty and there are bios in flight.  bio
* completion drops the count, maybe adds to the list, and wakes while
* holding the bio_lock so we don't need set_current_state()'s barrier
* and can call it after testing our condition.
*/
while (dio->refcount > 1 && dio->bio_list == NULL) {
  __set_current_state(TASK_UNINTERRUPTIBLE);
  dio->waiter = current;
  spin_unlock_irqrestore(&dio->bio_lock, flags);
  blk_io_schedule();
  /* wake up sets us TASK_RUNNING */
  spin_lock_irqsave(&dio->bio_lock, flags);
  dio->waiter = NULL;
}
if (dio->bio_list) {
  bio = dio->bio_list;
  dio->bio_list = bio->bi_private;
}
spin_unlock_irqrestore(&dio->bio_lock, flags);
return bio;
}

/*
* Process one completed BIO.  No locks are held.
*/
static blk_status_t dio_bio_complete(struct dio *dio, struct bio *bio)
{
blk_status_t err = bio->bi_status;
const enum req_op dio_op = dio->opf & REQ_OP_MASK;
bool should_dirty = dio_op == REQ_OP_READ && dio->should_dirty;

if (err) {
  if (err == BLK_STS_AGAIN && (bio->bi_opf & REQ_NOWAIT))
   dio->io_error = -EAGAIN;
  else
   dio->io_error = -EIO;
}

if (dio->is_async && should_dirty) {
  bio_check_pages_dirty(bio); /* transfers ownership */
} else {
  bio_release_pages(bio, should_dirty);
  bio_put(bio);
}
return err;
}

/*
* Wait on and process all in-flight BIOs.  This must only be called once
* all bios have been issued so that the refcount can only decrease.
* This just waits for all bios to make it through dio_bio_complete.  IO
* errors are propagated through dio->io_error and should be propagated via
* dio_complete().
*/
static void dio_await_completion(struct dio *dio)
{
struct bio *bio;
do {
  bio = dio_await_one(dio);
  if (bio)
   dio_bio_complete(dio, bio);
} while (bio);
}

/*
* A really large O_DIRECT read or write can generate a lot of BIOs.  So
* to keep the memory consumption sane we periodically reap any completed BIOs
* during the BIO generation phase.
*
* This also helps to limit the peak amount of pinned userspace memory.
*/
static inline int dio_bio_reap(struct dio *dio, struct dio_submit *sdio)
{
int ret = 0;

if (sdio->reap_counter++ >= 64) {
  while (dio->bio_list) {
   unsigned long flags;
   struct bio *bio;
   int ret2;

   spin_lock_irqsave(&dio->bio_lock, flags);
   bio = dio->bio_list;
   dio->bio_list = bio->bi_private;
   spin_unlock_irqrestore(&dio->bio_lock, flags);
   ret2 = blk_status_to_errno(dio_bio_complete(dio, bio));
   if (ret == 0)
    ret = ret2;
  }
  sdio->reap_counter = 0;
}
return ret;
}

static int dio_set_defer_completion(struct dio *dio)
{
struct super_block *sb = dio->inode->i_sb;

if (dio->defer_completion)
  return 0;
dio->defer_completion = true;
if (!sb->s_dio_done_wq)
  return sb_init_dio_done_wq(sb);
return 0;
}

/*
* Call into the fs to map some more disk blocks.  We record the current number
* of available blocks at sdio->blocks_available.  These are in units of the
* fs blocksize, i_blocksize(inode).
*
* The fs is allowed to map lots of blocks at once.  If it wants to do that,
* it uses the passed inode-relative block number as the file offset, as usual.
*
* get_block() is passed the number of i_blkbits-sized blocks which direct_io
* has remaining to do.  The fs should not map more than this number of blocks.
*
* If the fs has mapped a lot of blocks, it should populate bh->b_size to
* indicate how much contiguous disk space has been made available at
* bh->b_blocknr.
*
* If *any* of the mapped blocks are new, then the fs must set buffer_new().
* This isn't very efficient...
*
* In the case of filesystem holes: the fs may return an arbitrarily-large
* hole by returning an appropriate value in b_size and by clearing
* buffer_mapped().  However the direct-io code will only process holes one
* block at a time - it will repeatedly call get_block() as it walks the hole.
*/
static int get_more_blocks(struct dio *dio, struct dio_submit *sdio,
      struct buffer_head *map_bh)
{
const enum req_op dio_op = dio->opf & REQ_OP_MASK;
int ret;
sector_t fs_startblk; /* Into file, in filesystem-sized blocks */
sector_t fs_endblk; /* Into file, in filesystem-sized blocks */
unsigned long fs_count; /* Number of filesystem-sized blocks */
int create;
unsigned int i_blkbits = sdio->blkbits + sdio->blkfactor;
loff_t i_size;

/*
* If there was a memory error and we've overwritten all the
* mapped blocks then we can now return that memory error
*/
ret = dio->page_errors;
if (ret == 0) {
  BUG_ON(sdio->block_in_file >= sdio->final_block_in_request);
  fs_startblk = sdio->block_in_file >> sdio->blkfactor;
  fs_endblk = (sdio->final_block_in_request - 1) >>
     sdio->blkfactor;
  fs_count = fs_endblk - fs_startblk + 1;

  map_bh->b_state = 0;
  map_bh->b_size = fs_count << i_blkbits;

  /*
* For writes that could fill holes inside i_size on a
* DIO_SKIP_HOLES filesystem we forbid block creations: only
* overwrites are permitted. We will return early to the caller
* once we see an unmapped buffer head returned, and the caller
* will fall back to buffered I/O.
*
* Otherwise the decision is left to the get_blocks method,
* which may decide to handle it or also return an unmapped
* buffer head.
*/
  create = dio_op == REQ_OP_WRITE;
  if (dio->flags & DIO_SKIP_HOLES) {
   i_size = i_size_read(dio->inode);
   if (i_size && fs_startblk <= (i_size - 1) >> i_blkbits)
    create = 0;
  }

  ret = (*sdio->get_block)(dio->inode, fs_startblk,
      map_bh, create);

  /* Store for completion */
  dio->private = map_bh->b_private;

  if (ret == 0 && buffer_defer_completion(map_bh))
   ret = dio_set_defer_completion(dio);
}
return ret;
}

/*
* There is no bio.  Make one now.
*/
static inline int dio_new_bio(struct dio *dio, struct dio_submit *sdio,
  sector_t start_sector, struct buffer_head *map_bh)
{
sector_t sector;
int ret, nr_pages;

ret = dio_bio_reap(dio, sdio);
if (ret)
  goto out;
sector = start_sector << (sdio->blkbits - 9);
nr_pages = bio_max_segs(sdio->pages_in_io);
BUG_ON(nr_pages <= 0);
dio_bio_alloc(dio, sdio, map_bh->b_bdev, sector, nr_pages);
sdio->boundary = 0;
out:
return ret;
}

/*
* Attempt to put the current chunk of 'cur_page' into the current BIO.  If
* that was successful then update final_block_in_bio and take a ref against
* the just-added page.
*
* Return zero on success.  Non-zero means the caller needs to start a new BIO.
*/
static inline int dio_bio_add_page(struct dio *dio, struct dio_submit *sdio)
{
int ret;

ret = bio_add_page(sdio->bio, sdio->cur_page,
   sdio->cur_page_len, sdio->cur_page_offset);
if (ret == sdio->cur_page_len) {
  /*
* Decrement count only, if we are done with this page
*/
  if ((sdio->cur_page_len + sdio->cur_page_offset) == PAGE_SIZE)
   sdio->pages_in_io--;
  dio_pin_page(dio, sdio->cur_page);
  sdio->final_block_in_bio = sdio->cur_page_block +
   (sdio->cur_page_len >> sdio->blkbits);
  ret = 0;
} else {
  ret = 1;
}
return ret;
}

/*
* Put cur_page under IO.  The section of cur_page which is described by
* cur_page_offset,cur_page_len is put into a BIO.  The section of cur_page
* starts on-disk at cur_page_block.
*
* We take a ref against the page here (on behalf of its presence in the bio).
*
* The caller of this function is responsible for removing cur_page from the
* dio, and for dropping the refcount which came from that presence.
*/
static inline int dio_send_cur_page(struct dio *dio, struct dio_submit *sdio,
  struct buffer_head *map_bh)
{
int ret = 0;

if (sdio->bio) {
  loff_t cur_offset = sdio->cur_page_fs_offset;
  loff_t bio_next_offset = sdio->logical_offset_in_bio +
   sdio->bio->bi_iter.bi_size;

  /*
* See whether this new request is contiguous with the old.
*
* Btrfs cannot handle having logically non-contiguous requests
* submitted.  For example if you have
*
* Logical:  [0-4095][HOLE][8192-12287]
* Physical: [0-4095]      [4096-8191]
*
* We cannot submit those pages together as one BIO.  So if our
* current logical offset in the file does not equal what would
* be the next logical offset in the bio, submit the bio we
* have.
*/
  if (sdio->final_block_in_bio != sdio->cur_page_block ||
      cur_offset != bio_next_offset)
   dio_bio_submit(dio, sdio);
}

if (sdio->bio == NULL) {
  ret = dio_new_bio(dio, sdio, sdio->cur_page_block, map_bh);
  if (ret)
   goto out;
}

if (dio_bio_add_page(dio, sdio) != 0) {
  dio_bio_submit(dio, sdio);
  ret = dio_new_bio(dio, sdio, sdio->cur_page_block, map_bh);
  if (ret == 0) {
   ret = dio_bio_add_page(dio, sdio);
   BUG_ON(ret != 0);
  }
}
out:
return ret;
}

/*
* An autonomous function to put a chunk of a page under deferred IO.
*
* The caller doesn't actually know (or care) whether this piece of page is in
* a BIO, or is under IO or whatever.  We just take care of all possible
* situations here.  The separation between the logic of do_direct_IO() and
* that of submit_page_section() is important for clarity.  Please don't break.
*
* The chunk of page starts on-disk at blocknr.
*
* We perform deferred IO, by recording the last-submitted page inside our
* private part of the dio structure.  If possible, we just expand the IO
* across that page here.
*
* If that doesn't work out then we put the old page into the bio and add this
* page to the dio instead.
*/
static inline int
submit_page_section(struct dio *dio, struct dio_submit *sdio, struct page *page,
      unsigned offset, unsigned len, sector_t blocknr,
      struct buffer_head *map_bh)
{
const enum req_op dio_op = dio->opf & REQ_OP_MASK;
int ret = 0;
int boundary = sdio->boundary; /* dio_send_cur_page may clear it */

if (dio_op == REQ_OP_WRITE) {
  /*
* Read accounting is performed in submit_bio()
*/
  task_io_account_write(len);
}

/*
* Can we just grow the current page's presence in the dio?
*/
if (sdio->cur_page == page &&
     sdio->cur_page_offset + sdio->cur_page_len == offset &&
     sdio->cur_page_block +
     (sdio->cur_page_len >> sdio->blkbits) == blocknr) {
  sdio->cur_page_len += len;
  goto out;
}

/*
* If there's a deferred page already there then send it.
*/
if (sdio->cur_page) {
  ret = dio_send_cur_page(dio, sdio, map_bh);
  dio_unpin_page(dio, sdio->cur_page);
  sdio->cur_page = NULL;
  if (ret)
   return ret;
}

dio_pin_page(dio, page);  /* It is in dio */
sdio->cur_page = page;
sdio->cur_page_offset = offset;
sdio->cur_page_len = len;
sdio->cur_page_block = blocknr;
sdio->cur_page_fs_offset = sdio->block_in_file << sdio->blkbits;
out:
/*
* If boundary then we want to schedule the IO now to
* avoid metadata seeks.
*/
if (boundary) {
  ret = dio_send_cur_page(dio, sdio, map_bh);
  if (sdio->bio)
   dio_bio_submit(dio, sdio);
  dio_unpin_page(dio, sdio->cur_page);
  sdio->cur_page = NULL;
}
return ret;
}

/*
* If we are not writing the entire block and get_block() allocated
* the block for us, we need to fill-in the unused portion of the
* block with zeros. This happens only if user-buffer, fileoffset or
* io length is not filesystem block-size multiple.
*
* `end' is zero if we're doing the start of the IO, 1 at the end of the
* IO.
*/
static inline void dio_zero_block(struct dio *dio, struct dio_submit *sdio,
  int end, struct buffer_head *map_bh)
{
unsigned dio_blocks_per_fs_block;
unsigned this_chunk_blocks; /* In dio_blocks */
unsigned this_chunk_bytes;
struct page *page;

sdio->start_zero_done = 1;
if (!sdio->blkfactor || !buffer_new(map_bh))
  return;

dio_blocks_per_fs_block = 1 << sdio->blkfactor;
this_chunk_blocks = sdio->block_in_file & (dio_blocks_per_fs_block - 1);

if (!this_chunk_blocks)
  return;

/*
* We need to zero out part of an fs block.  It is either at the
* beginning or the end of the fs block.
*/
if (end)
  this_chunk_blocks = dio_blocks_per_fs_block - this_chunk_blocks;

this_chunk_bytes = this_chunk_blocks << sdio->blkbits;

page = ZERO_PAGE(0);
if (submit_page_section(dio, sdio, page, 0, this_chunk_bytes,
    sdio->next_block_for_io, map_bh))
  return;

sdio->next_block_for_io += this_chunk_blocks;
}

/*
* Walk the user pages, and the file, mapping blocks to disk and generating
* a sequence of (page,offset,len,block) mappings.  These mappings are injected
* into submit_page_section(), which takes care of the next stage of submission
*
* Direct IO against a blockdev is different from a file.  Because we can
* happily perform page-sized but 512-byte aligned IOs.  It is important that
* blockdev IO be able to have fine alignment and large sizes.
*
* So what we do is to permit the ->get_block function to populate bh.b_size
* with the size of IO which is permitted at this offset and this i_blkbits.
*
* For best results, the blockdev should be set up with 512-byte i_blkbits and
* it should set b_size to PAGE_SIZE or more inside get_block().  This gives
* fine alignment but still allows this function to work in PAGE_SIZE units.
*/
static int do_direct_IO(struct dio *dio, struct dio_submit *sdio,
   struct buffer_head *map_bh)
{
const enum req_op dio_op = dio->opf & REQ_OP_MASK;
const unsigned blkbits = sdio->blkbits;
const unsigned i_blkbits = blkbits + sdio->blkfactor;
int ret = 0;

while (sdio->block_in_file < sdio->final_block_in_request) {
  struct page *page;
  size_t from, to;

  page = dio_get_page(dio, sdio);
  if (IS_ERR(page)) {
   ret = PTR_ERR(page);
   goto out;
  }
  from = sdio->head ? 0 : sdio->from;
  to = (sdio->head == sdio->tail - 1) ? sdio->to : PAGE_SIZE;
  sdio->head++;

  while (from < to) {
   unsigned this_chunk_bytes; /* # of bytes mapped */
   unsigned this_chunk_blocks; /* # of blocks */
   unsigned u;

   if (sdio->blocks_available == 0) {
    /*
* Need to go and map some more disk
*/
    unsigned long blkmask;
    unsigned long dio_remainder;

    ret = get_more_blocks(dio, sdio, map_bh);
    if (ret) {
     dio_unpin_page(dio, page);
     goto out;
    }
    if (!buffer_mapped(map_bh))
     goto do_holes;

    sdio->blocks_available =
      map_bh->b_size >> blkbits;
    sdio->next_block_for_io =
     map_bh->b_blocknr << sdio->blkfactor;
    if (buffer_new(map_bh)) {
     clean_bdev_aliases(
      map_bh->b_bdev,
      map_bh->b_blocknr,
      map_bh->b_size >> i_blkbits);
    }

    if (!sdio->blkfactor)
     goto do_holes;

    blkmask = (1 << sdio->blkfactor) - 1;
    dio_remainder = (sdio->block_in_file & blkmask);

    /*
* If we are at the start of IO and that IO
* starts partway into a fs-block,
* dio_remainder will be non-zero.  If the IO
* is a read then we can simply advance the IO
* cursor to the first block which is to be
* read.  But if the IO is a write and the
* block was newly allocated we cannot do that;
* the start of the fs block must be zeroed out
* on-disk
*/
    if (!buffer_new(map_bh))
     sdio->next_block_for_io += dio_remainder;
    sdio->blocks_available -= dio_remainder;
   }
do_holes:
   /* Handle holes */
   if (!buffer_mapped(map_bh)) {
    loff_t i_size_aligned;

    /* AKPM: eargh, -ENOTBLK is a hack */
    if (dio_op == REQ_OP_WRITE) {
     dio_unpin_page(dio, page);
     return -ENOTBLK;
    }

    /*
* Be sure to account for a partial block as the
* last block in the file
*/
    i_size_aligned = ALIGN(i_size_read(dio->inode),
       1 << blkbits);
    if (sdio->block_in_file >=
      i_size_aligned >> blkbits) {
     /* We hit eof */
     dio_unpin_page(dio, page);
     goto out;
    }
    memzero_page(page, from, 1 << blkbits);
    sdio->block_in_file++;
    from += 1 << blkbits;
    dio->result += 1 << blkbits;
    goto next_block;
   }

   /*
* If we're performing IO which has an alignment which
* is finer than the underlying fs, go check to see if
* we must zero out the start of this block.
*/
   if (unlikely(sdio->blkfactor && !sdio->start_zero_done))
    dio_zero_block(dio, sdio, 0, map_bh);

   /*
* Work out, in this_chunk_blocks, how much disk we
* can add to this page
*/
   this_chunk_blocks = sdio->blocks_available;
   u = (to - from) >> blkbits;
   if (this_chunk_blocks > u)
    this_chunk_blocks = u;
   u = sdio->final_block_in_request - sdio->block_in_file;
   if (this_chunk_blocks > u)
    this_chunk_blocks = u;
   this_chunk_bytes = this_chunk_blocks << blkbits;
   BUG_ON(this_chunk_bytes == 0);

   if (this_chunk_blocks == sdio->blocks_available)
    sdio->boundary = buffer_boundary(map_bh);
   ret = submit_page_section(dio, sdio, page,
        from,
        this_chunk_bytes,
        sdio->next_block_for_io,
        map_bh);
   if (ret) {
    dio_unpin_page(dio, page);
    goto out;
   }
   sdio->next_block_for_io += this_chunk_blocks;

   sdio->block_in_file += this_chunk_blocks;
   from += this_chunk_bytes;
   dio->result += this_chunk_bytes;
   sdio->blocks_available -= this_chunk_blocks;
next_block:
   BUG_ON(sdio->block_in_file > sdio->final_block_in_request);
   if (sdio->block_in_file == sdio->final_block_in_request)
    break;
  }

  /* Drop the pin which was taken in get_user_pages() */
  dio_unpin_page(dio, page);
}
out:
return ret;
}

static inline int drop_refcount(struct dio *dio)
{
int ret2;
unsigned long flags;

/*
* Sync will always be dropping the final ref and completing the
* operation.  AIO can if it was a broken operation described above or
* in fact if all the bios race to complete before we get here.  In
* that case dio_complete() translates the EIOCBQUEUED into the proper
* return code that the caller will hand to ->complete().
*
* This is managed by the bio_lock instead of being an atomic_t so that
* completion paths can drop their ref and use the remaining count to
* decide to wake the submission path atomically.
*/
spin_lock_irqsave(&dio->bio_lock, flags);
ret2 = --dio->refcount;
spin_unlock_irqrestore(&dio->bio_lock, flags);
return ret2;
}

/*
* This is a library function for use by filesystem drivers.
*
* The locking rules are governed by the flags parameter:
*  - if the flags value contains DIO_LOCKING we use a fancy locking
*    scheme for dumb filesystems.
*    For writes this function is called under i_rwsem and returns with
*    i_rwsem held, for reads, i_rwsem is not held on entry, but it is
*    taken and dropped again before returning.
*  - if the flags value does NOT contain DIO_LOCKING we don't use any
*    internal locking but rather rely on the filesystem to synchronize
*    direct I/O reads/writes versus each other and truncate.
*
* To help with locking against truncate we incremented the i_dio_count
* counter before starting direct I/O, and decrement it once we are done.
* Truncate can wait for it to reach zero to provide exclusion.  It is
* expected that filesystem provide exclusion between new direct I/O
* and truncates.  For DIO_LOCKING filesystems this is done by i_rwsem,
* but other filesystems need to take care of this on their own.
*
* NOTE: if you pass "sdio" to anything by pointer make sure that function
* is always inlined. Otherwise gcc is unable to split the structure into
* individual fields and will generate much worse code. This is important
* for the whole file.
*/
ssize_t __blockdev_direct_IO(struct kiocb *iocb, struct inode *inode,
  struct block_device *bdev, struct iov_iter *iter,
  get_block_t get_block, dio_iodone_t end_io,
  int flags)
{
unsigned i_blkbits = READ_ONCE(inode->i_blkbits);
unsigned blkbits = i_blkbits;
unsigned blocksize_mask = (1 << blkbits) - 1;
ssize_t retval = -EINVAL;
const size_t count = iov_iter_count(iter);
loff_t offset = iocb->ki_pos;
const loff_t end = offset + count;
struct dio *dio;
struct dio_submit sdio = { NULL, };
struct buffer_head map_bh = { 0, };
struct blk_plug plug;
unsigned long align = offset | iov_iter_alignment(iter);

/* watch out for a 0 len io from a tricksy fs */
if (iov_iter_rw(iter) == READ && !count)
  return 0;

dio = kmem_cache_alloc(dio_cache, GFP_KERNEL);
if (!dio)
  return -ENOMEM;
/*
* Believe it or not, zeroing out the page array caused a .5%
* performance regression in a database benchmark.  So, we take
* care to only zero out what's needed.
*/
memset(dio, 0, offsetof(struct dio, pages));

dio->flags = flags;
if (dio->flags & DIO_LOCKING && iov_iter_rw(iter) == READ) {
  /* will be released by direct_io_worker */
  inode_lock(inode);
}
dio->is_pinned = iov_iter_extract_will_pin(iter);

/* Once we sampled i_size check for reads beyond EOF */
dio->i_size = i_size_read(inode);
if (iov_iter_rw(iter) == READ && offset >= dio->i_size) {
  retval = 0;
  goto fail_dio;
}

if (align & blocksize_mask) {
  if (bdev)
   blkbits = blksize_bits(bdev_logical_block_size(bdev));
  blocksize_mask = (1 << blkbits) - 1;
  if (align & blocksize_mask)
   goto fail_dio;
}

if (dio->flags & DIO_LOCKING && iov_iter_rw(iter) == READ) {
  struct address_space *mapping = iocb->ki_filp->f_mapping;

  retval = filemap_write_and_wait_range(mapping, offset, end - 1);
  if (retval)
   goto fail_dio;
}

/*
* For file extending writes updating i_size before data writeouts
* complete can expose uninitialized blocks in dumb filesystems.
* In that case we need to wait for I/O completion even if asked
* for an asynchronous write.
*/
if (is_sync_kiocb(iocb))
  dio->is_async = false;
else if (iov_iter_rw(iter) == WRITE && end > i_size_read(inode))
  dio->is_async = false;
else
  dio->is_async = true;

dio->inode = inode;
if (iov_iter_rw(iter) == WRITE) {
  dio->opf = REQ_OP_WRITE | REQ_SYNC | REQ_IDLE;
  if (iocb->ki_flags & IOCB_NOWAIT)
   dio->opf |= REQ_NOWAIT;
} else {
  dio->opf = REQ_OP_READ;
}

/*
* For AIO O_(D)SYNC writes we need to defer completions to a workqueue
* so that we can call ->fsync.
*/
if (dio->is_async && iov_iter_rw(iter) == WRITE) {
  retval = 0;
  if (iocb_is_dsync(iocb))
   retval = dio_set_defer_completion(dio);
  else if (!dio->inode->i_sb->s_dio_done_wq) {
   /*
* In case of AIO write racing with buffered read we
* need to defer completion. We can't decide this now,
* however the workqueue needs to be initialized here.
*/
   retval = sb_init_dio_done_wq(dio->inode->i_sb);
  }
  if (retval)
   goto fail_dio;
}

/*
* Will be decremented at I/O completion time.
*/
inode_dio_begin(inode);

sdio.blkbits = blkbits;
sdio.blkfactor = i_blkbits - blkbits;
sdio.block_in_file = offset >> blkbits;

sdio.get_block = get_block;
dio->end_io = end_io;
sdio.final_block_in_bio = -1;
sdio.next_block_for_io = -1;

dio->iocb = iocb;

spin_lock_init(&dio->bio_lock);
dio->refcount = 1;

dio->should_dirty = user_backed_iter(iter) && iov_iter_rw(iter) == READ;
sdio.iter = iter;
sdio.final_block_in_request = end >> blkbits;

/*
* In case of non-aligned buffers, we may need 2 more
* pages since we need to zero out first and last block.
*/
if (unlikely(sdio.blkfactor))
  sdio.pages_in_io = 2;

sdio.pages_in_io += iov_iter_npages(iter, INT_MAX);

blk_start_plug(&plug);

retval = do_direct_IO(dio, &sdio, &map_bh);
if (retval)
  dio_cleanup(dio, &sdio);

if (retval == -ENOTBLK) {
  /*
* The remaining part of the request will be
* handled by buffered I/O when we return
*/
  retval = 0;
}
/*
* There may be some unwritten disk at the end of a part-written
* fs-block-sized block.  Go zero that now.
*/
dio_zero_block(dio, &sdio, 1, &map_bh);

if (sdio.cur_page) {
  ssize_t ret2;

  ret2 = dio_send_cur_page(dio, &sdio, &map_bh);
  if (retval == 0)
   retval = ret2;
  dio_unpin_page(dio, sdio.cur_page);
  sdio.cur_page = NULL;
}
if (sdio.bio)
  dio_bio_submit(dio, &sdio);

blk_finish_plug(&plug);

/*
* It is possible that, we return short IO due to end of file.
* In that case, we need to release all the pages we got hold on.
*/
dio_cleanup(dio, &sdio);

/*
* All block lookups have been performed. For READ requests
* we can let i_rwsem go now that its achieved its purpose
* of protecting us from looking up uninitialized blocks.
*/
if (iov_iter_rw(iter) == READ && (dio->flags & DIO_LOCKING))
  inode_unlock(dio->inode);

/*
* The only time we want to leave bios in flight is when a successful
* partial aio read or full aio write have been setup.  In that case
* bio completion will call aio_complete.  The only time it's safe to
* call aio_complete is when we return -EIOCBQUEUED, so we key on that.
* This had *better* be the only place that raises -EIOCBQUEUED.
*/
BUG_ON(retval == -EIOCBQUEUED);
if (dio->is_async && retval == 0 && dio->result &&
     (iov_iter_rw(iter) == READ || dio->result == count))
  retval = -EIOCBQUEUED;
else
  dio_await_completion(dio);

if (drop_refcount(dio) == 0) {
  retval = dio_complete(dio, retval, DIO_COMPLETE_INVALIDATE);
} else
  BUG_ON(retval != -EIOCBQUEUED);

return retval;

fail_dio:
if (dio->flags & DIO_LOCKING && iov_iter_rw(iter) == READ)
  inode_unlock(inode);

kmem_cache_free(dio_cache, dio);
return retval;
}
EXPORT_SYMBOL(__blockdev_direct_IO);

static __init int dio_init(void)
{
dio_cache = KMEM_CACHE(dio, SLAB_PANIC);
return 0;
}
module_init(dio_init)

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