Quelle mpage.c Sprache: C

// SPDX-License-Identifier: GPL-2.0
/*
* fs/mpage.c
*
* Copyright (C) 2002, Linus Torvalds.
*
* Contains functions related to preparing and submitting BIOs which contain
* multiple pagecache pages.
*
* 15May2002 Andrew Morton
* Initial version
* 27Jun2002 axboe@suse.de
* use bio_add_page() to build bio's just the right size
*/

#include <linux/kernel.h>
#include <linux/export.h>
#include <linux/mm.h>
#include <linux/kdev_t.h>
#include <linux/gfp.h>
#include <linux/bio.h>
#include <linux/fs.h>
#include <linux/buffer_head.h>
#include <linux/blkdev.h>
#include <linux/highmem.h>
#include <linux/prefetch.h>
#include <linux/mpage.h>
#include <linux/mm_inline.h>
#include <linux/writeback.h>
#include <linux/backing-dev.h>
#include <linux/pagevec.h>
#include "internal.h"

/*
* I/O completion handler for multipage BIOs.
*
* The mpage code never puts partial pages into a BIO (except for end-of-file).
* If a page does not map to a contiguous run of blocks then it simply falls
* back to block_read_full_folio().
*
* Why is this?  If a page's completion depends on a number of different BIOs
* which can complete in any order (or at the same time) then determining the
* status of that page is hard.  See end_buffer_async_read() for the details.
* There is no point in duplicating all that complexity.
*/
static void mpage_read_end_io(struct bio *bio)
{
struct folio_iter fi;
int err = blk_status_to_errno(bio->bi_status);

bio_for_each_folio_all(fi, bio)
  folio_end_read(fi.folio, err == 0);

bio_put(bio);
}

static void mpage_write_end_io(struct bio *bio)
{
struct folio_iter fi;
int err = blk_status_to_errno(bio->bi_status);

bio_for_each_folio_all(fi, bio) {
  if (err)
   mapping_set_error(fi.folio->mapping, err);
  folio_end_writeback(fi.folio);
}

bio_put(bio);
}

static struct bio *mpage_bio_submit_read(struct bio *bio)
{
bio->bi_end_io = mpage_read_end_io;
guard_bio_eod(bio);
submit_bio(bio);
return NULL;
}

static struct bio *mpage_bio_submit_write(struct bio *bio)
{
bio->bi_end_io = mpage_write_end_io;
guard_bio_eod(bio);
submit_bio(bio);
return NULL;
}

/*
* support function for mpage_readahead.  The fs supplied get_block might
* return an up to date buffer.  This is used to map that buffer into
* the page, which allows read_folio to avoid triggering a duplicate call
* to get_block.
*
* The idea is to avoid adding buffers to pages that don't already have
* them.  So when the buffer is up to date and the page size == block size,
* this marks the page up to date instead of adding new buffers.
*/
static void map_buffer_to_folio(struct folio *folio, struct buffer_head *bh,
  int page_block)
{
struct inode *inode = folio->mapping->host;
struct buffer_head *page_bh, *head;
int block = 0;

head = folio_buffers(folio);
if (!head) {
  /*
* don't make any buffers if there is only one buffer on
* the folio and the folio just needs to be set up to date
*/
  if (inode->i_blkbits == folio_shift(folio) &&
      buffer_uptodate(bh)) {
   folio_mark_uptodate(folio);
   return;
  }
  head = create_empty_buffers(folio, i_blocksize(inode), 0);
}

page_bh = head;
do {
  if (block == page_block) {
   page_bh->b_state = bh->b_state;
   page_bh->b_bdev = bh->b_bdev;
   page_bh->b_blocknr = bh->b_blocknr;
   break;
  }
  page_bh = page_bh->b_this_page;
  block++;
} while (page_bh != head);
}

struct mpage_readpage_args {
struct bio *bio;
struct folio *folio;
unsigned int nr_pages;
bool is_readahead;
sector_t last_block_in_bio;
struct buffer_head map_bh;
unsigned long first_logical_block;
get_block_t *get_block;
};

/*
* This is the worker routine which does all the work of mapping the disk
* blocks and constructs largest possible bios, submits them for IO if the
* blocks are not contiguous on the disk.
*
* We pass a buffer_head back and forth and use its buffer_mapped() flag to
* represent the validity of its disk mapping and to decide when to do the next
* get_block() call.
*/
static struct bio *do_mpage_readpage(struct mpage_readpage_args *args)
{
struct folio *folio = args->folio;
struct inode *inode = folio->mapping->host;
const unsigned blkbits = inode->i_blkbits;
const unsigned blocks_per_folio = folio_size(folio) >> blkbits;
const unsigned blocksize = 1 << blkbits;
struct buffer_head *map_bh = &args->map_bh;
sector_t block_in_file;
sector_t last_block;
sector_t last_block_in_file;
sector_t first_block;
unsigned page_block;
unsigned first_hole = blocks_per_folio;
struct block_device *bdev = NULL;
int length;
int fully_mapped = 1;
blk_opf_t opf = REQ_OP_READ;
unsigned nblocks;
unsigned relative_block;
gfp_t gfp = mapping_gfp_constraint(folio->mapping, GFP_KERNEL);

if (args->is_readahead) {
  opf |= REQ_RAHEAD;
  gfp |= __GFP_NORETRY | __GFP_NOWARN;
}

if (folio_buffers(folio))
  goto confused;

block_in_file = folio_pos(folio) >> blkbits;
last_block = block_in_file + ((args->nr_pages * PAGE_SIZE) >> blkbits);
last_block_in_file = (i_size_read(inode) + blocksize - 1) >> blkbits;
if (last_block > last_block_in_file)
  last_block = last_block_in_file;
page_block = 0;

/*
* Map blocks using the result from the previous get_blocks call first.
*/
nblocks = map_bh->b_size >> blkbits;
if (buffer_mapped(map_bh) &&
   block_in_file > args->first_logical_block &&
   block_in_file < (args->first_logical_block + nblocks)) {
  unsigned map_offset = block_in_file - args->first_logical_block;
  unsigned last = nblocks - map_offset;

  first_block = map_bh->b_blocknr + map_offset;
  for (relative_block = 0; ; relative_block++) {
   if (relative_block == last) {
    clear_buffer_mapped(map_bh);
    break;
   }
   if (page_block == blocks_per_folio)
    break;
   page_block++;
   block_in_file++;
  }
  bdev = map_bh->b_bdev;
}

/*
* Then do more get_blocks calls until we are done with this folio.
*/
map_bh->b_folio = folio;
while (page_block < blocks_per_folio) {
  map_bh->b_state = 0;
  map_bh->b_size = 0;

  if (block_in_file < last_block) {
   map_bh->b_size = (last_block-block_in_file) << blkbits;
   if (args->get_block(inode, block_in_file, map_bh, 0))
    goto confused;
   args->first_logical_block = block_in_file;
  }

  if (!buffer_mapped(map_bh)) {
   fully_mapped = 0;
   if (first_hole == blocks_per_folio)
    first_hole = page_block;
   page_block++;
   block_in_file++;
   continue;
  }

  /* some filesystems will copy data into the page during
* the get_block call, in which case we don't want to
* read it again.  map_buffer_to_folio copies the data
* we just collected from get_block into the folio's buffers
* so read_folio doesn't have to repeat the get_block call
*/
  if (buffer_uptodate(map_bh)) {
   map_buffer_to_folio(folio, map_bh, page_block);
   goto confused;
  }

  if (first_hole != blocks_per_folio)
   goto confused;  /* hole -> non-hole */

  /* Contiguous blocks? */
  if (!page_block)
   first_block = map_bh->b_blocknr;
  else if (first_block + page_block != map_bh->b_blocknr)
   goto confused;
  nblocks = map_bh->b_size >> blkbits;
  for (relative_block = 0; ; relative_block++) {
   if (relative_block == nblocks) {
    clear_buffer_mapped(map_bh);
    break;
   } else if (page_block == blocks_per_folio)
    break;
   page_block++;
   block_in_file++;
  }
  bdev = map_bh->b_bdev;
}

if (first_hole != blocks_per_folio) {
  folio_zero_segment(folio, first_hole << blkbits, folio_size(folio));
  if (first_hole == 0) {
   folio_mark_uptodate(folio);
   folio_unlock(folio);
   goto out;
  }
} else if (fully_mapped) {
  folio_set_mappedtodisk(folio);
}

/*
* This folio will go to BIO.  Do we need to send this BIO off first?
*/
if (args->bio && (args->last_block_in_bio != first_block - 1))
  args->bio = mpage_bio_submit_read(args->bio);

alloc_new:
if (args->bio == NULL) {
  args->bio = bio_alloc(bdev, bio_max_segs(args->nr_pages), opf,
          gfp);
  if (args->bio == NULL)
   goto confused;
  args->bio->bi_iter.bi_sector = first_block << (blkbits - 9);
}

length = first_hole << blkbits;
if (!bio_add_folio(args->bio, folio, length, 0)) {
  args->bio = mpage_bio_submit_read(args->bio);
  goto alloc_new;
}

relative_block = block_in_file - args->first_logical_block;
nblocks = map_bh->b_size >> blkbits;
if ((buffer_boundary(map_bh) && relative_block == nblocks) ||
     (first_hole != blocks_per_folio))
  args->bio = mpage_bio_submit_read(args->bio);
else
  args->last_block_in_bio = first_block + blocks_per_folio - 1;
out:
return args->bio;

confused:
if (args->bio)
  args->bio = mpage_bio_submit_read(args->bio);
if (!folio_test_uptodate(folio))
  block_read_full_folio(folio, args->get_block);
else
  folio_unlock(folio);
goto out;
}

/**
* mpage_readahead - start reads against pages
* @rac: Describes which pages to read.
* @get_block: The filesystem's block mapper function.
*
* This function walks the pages and the blocks within each page, building and
* emitting large BIOs.
*
* If anything unusual happens, such as:
*
* - encountering a page which has buffers
* - encountering a page which has a non-hole after a hole
* - encountering a page with non-contiguous blocks
*
* then this code just gives up and calls the buffer_head-based read function.
* It does handle a page which has holes at the end - that is a common case:
* the end-of-file on blocksize < PAGE_SIZE setups.
*
* BH_Boundary explanation:
*
* There is a problem.  The mpage read code assembles several pages, gets all
* their disk mappings, and then submits them all.  That's fine, but obtaining
* the disk mappings may require I/O.  Reads of indirect blocks, for example.
*
* So an mpage read of the first 16 blocks of an ext2 file will cause I/O to be
* submitted in the following order:
*
* 12 0 1 2 3 4 5 6 7 8 9 10 11 13 14 15 16
*
* because the indirect block has to be read to get the mappings of blocks
* 13,14,15,16.  Obviously, this impacts performance.
*
* So what we do it to allow the filesystem's get_block() function to set
* BH_Boundary when it maps block 11.  BH_Boundary says: mapping of the block
* after this one will require I/O against a block which is probably close to
* this one.  So you should push what I/O you have currently accumulated.
*
* This all causes the disk requests to be issued in the correct order.
*/
void mpage_readahead(struct readahead_control *rac, get_block_t get_block)
{
struct folio *folio;
struct mpage_readpage_args args = {
  .get_block = get_block,
  .is_readahead = true,
};

while ((folio = readahead_folio(rac))) {
  prefetchw(&folio->flags);
  args.folio = folio;
  args.nr_pages = readahead_count(rac);
  args.bio = do_mpage_readpage(&args);
}
if (args.bio)
  mpage_bio_submit_read(args.bio);
}
EXPORT_SYMBOL(mpage_readahead);

/*
* This isn't called much at all
*/
int mpage_read_folio(struct folio *folio, get_block_t get_block)
{
struct mpage_readpage_args args = {
  .folio = folio,
  .nr_pages = folio_nr_pages(folio),
  .get_block = get_block,
};

args.bio = do_mpage_readpage(&args);
if (args.bio)
  mpage_bio_submit_read(args.bio);
return 0;
}
EXPORT_SYMBOL(mpage_read_folio);

/*
* Writing is not so simple.
*
* If the page has buffers then they will be used for obtaining the disk
* mapping.  We only support pages which are fully mapped-and-dirty, with a
* special case for pages which are unmapped at the end: end-of-file.
*
* If the page has no buffers (preferred) then the page is mapped here.
*
* If all blocks are found to be contiguous then the page can go into the
* BIO.  Otherwise fall back to the mapping's writepage().
*
* FIXME: This code wants an estimate of how many pages are still to be
* written, so it can intelligently allocate a suitably-sized BIO.  For now,
* just allocate full-size (16-page) BIOs.
*/

struct mpage_data {
struct bio *bio;
sector_t last_block_in_bio;
get_block_t *get_block;
};

/*
* We have our BIO, so we can now mark the buffers clean.  Make
* sure to only clean buffers which we know we'll be writing.
*/
static void clean_buffers(struct folio *folio, unsigned first_unmapped)
{
unsigned buffer_counter = 0;
struct buffer_head *bh, *head = folio_buffers(folio);

if (!head)
  return;
bh = head;

do {
  if (buffer_counter++ == first_unmapped)
   break;
  clear_buffer_dirty(bh);
  bh = bh->b_this_page;
} while (bh != head);

/*
* we cannot drop the bh if the page is not uptodate or a concurrent
* read_folio would fail to serialize with the bh and it would read from
* disk before we reach the platter.
*/
if (buffer_heads_over_limit && folio_test_uptodate(folio))
  try_to_free_buffers(folio);
}

static int mpage_write_folio(struct writeback_control *wbc, struct folio *folio,
  struct mpage_data *mpd)
{
struct bio *bio = mpd->bio;
struct address_space *mapping = folio->mapping;
struct inode *inode = mapping->host;
const unsigned blkbits = inode->i_blkbits;
const unsigned blocks_per_folio = folio_size(folio) >> blkbits;
sector_t last_block;
sector_t block_in_file;
sector_t first_block;
unsigned page_block;
unsigned first_unmapped = blocks_per_folio;
struct block_device *bdev = NULL;
int boundary = 0;
sector_t boundary_block = 0;
struct block_device *boundary_bdev = NULL;
size_t length;
struct buffer_head map_bh;
loff_t i_size = i_size_read(inode);
int ret = 0;
struct buffer_head *head = folio_buffers(folio);

if (head) {
  struct buffer_head *bh = head;

  /* If they're all mapped and dirty, do it */
  page_block = 0;
  do {
   BUG_ON(buffer_locked(bh));
   if (!buffer_mapped(bh)) {
    /*
* unmapped dirty buffers are created by
* block_dirty_folio -> mmapped data
*/
    if (buffer_dirty(bh))
     goto confused;
    if (first_unmapped == blocks_per_folio)
     first_unmapped = page_block;
    continue;
   }

   if (first_unmapped != blocks_per_folio)
    goto confused; /* hole -> non-hole */

   if (!buffer_dirty(bh) || !buffer_uptodate(bh))
    goto confused;
   if (page_block) {
    if (bh->b_blocknr != first_block + page_block)
     goto confused;
   } else {
    first_block = bh->b_blocknr;
   }
   page_block++;
   boundary = buffer_boundary(bh);
   if (boundary) {
    boundary_block = bh->b_blocknr;
    boundary_bdev = bh->b_bdev;
   }
   bdev = bh->b_bdev;
  } while ((bh = bh->b_this_page) != head);

  if (first_unmapped)
   goto page_is_mapped;

  /*
* Page has buffers, but they are all unmapped. The page was
* created by pagein or read over a hole which was handled by
* block_read_full_folio().  If this address_space is also
* using mpage_readahead then this can rarely happen.
*/
  goto confused;
}

/*
* The page has no buffers: map it to disk
*/
BUG_ON(!folio_test_uptodate(folio));
block_in_file = folio_pos(folio) >> blkbits;
/*
* Whole page beyond EOF? Skip allocating blocks to avoid leaking
* space.
*/
if (block_in_file >= (i_size + (1 << blkbits) - 1) >> blkbits)
  goto page_is_mapped;
last_block = (i_size - 1) >> blkbits;
map_bh.b_folio = folio;
for (page_block = 0; page_block < blocks_per_folio; ) {

  map_bh.b_state = 0;
  map_bh.b_size = 1 << blkbits;
  if (mpd->get_block(inode, block_in_file, &map_bh, 1))
   goto confused;
  if (!buffer_mapped(&map_bh))
   goto confused;
  if (buffer_new(&map_bh))
   clean_bdev_bh_alias(&map_bh);
  if (buffer_boundary(&map_bh)) {
   boundary_block = map_bh.b_blocknr;
   boundary_bdev = map_bh.b_bdev;
  }
  if (page_block) {
   if (map_bh.b_blocknr != first_block + page_block)
    goto confused;
  } else {
   first_block = map_bh.b_blocknr;
  }
  page_block++;
  boundary = buffer_boundary(&map_bh);
  bdev = map_bh.b_bdev;
  if (block_in_file == last_block)
   break;
  block_in_file++;
}
BUG_ON(page_block == 0);

first_unmapped = page_block;

page_is_mapped:
/* Don't bother writing beyond EOF, truncate will discard the folio */
if (folio_pos(folio) >= i_size)
  goto confused;
length = folio_size(folio);
if (folio_pos(folio) + length > i_size) {
  /*
* The page straddles i_size.  It must be zeroed out on each
* and every writepage invocation because it may be mmapped.
* "A file is mapped in multiples of the page size.  For a file
* that is not a multiple of the page size, the remaining memory
* is zeroed when mapped, and writes to that region are not
* written out to the file."
*/
  length = i_size - folio_pos(folio);
  folio_zero_segment(folio, length, folio_size(folio));
}

/*
* This page will go to BIO.  Do we need to send this BIO off first?
*/
if (bio && mpd->last_block_in_bio != first_block - 1)
  bio = mpage_bio_submit_write(bio);

alloc_new:
if (bio == NULL) {
  bio = bio_alloc(bdev, BIO_MAX_VECS,
    REQ_OP_WRITE | wbc_to_write_flags(wbc),
    GFP_NOFS);
  bio->bi_iter.bi_sector = first_block << (blkbits - 9);
  wbc_init_bio(wbc, bio);
  bio->bi_write_hint = inode->i_write_hint;
}

/*
* Must try to add the page before marking the buffer clean or
* the confused fail path above (OOM) will be very confused when
* it finds all bh marked clean (i.e. it will not write anything)
*/
wbc_account_cgroup_owner(wbc, folio, folio_size(folio));
length = first_unmapped << blkbits;
if (!bio_add_folio(bio, folio, length, 0)) {
  bio = mpage_bio_submit_write(bio);
  goto alloc_new;
}

clean_buffers(folio, first_unmapped);

BUG_ON(folio_test_writeback(folio));
folio_start_writeback(folio);
folio_unlock(folio);
if (boundary || (first_unmapped != blocks_per_folio)) {
  bio = mpage_bio_submit_write(bio);
  if (boundary_block) {
   write_boundary_block(boundary_bdev,
     boundary_block, 1 << blkbits);
  }
} else {
  mpd->last_block_in_bio = first_block + blocks_per_folio - 1;
}
goto out;

confused:
if (bio)
  bio = mpage_bio_submit_write(bio);

/*
* The caller has a ref on the inode, so *mapping is stable
*/
ret = block_write_full_folio(folio, wbc, mpd->get_block);
mapping_set_error(mapping, ret);
out:
mpd->bio = bio;
return ret;
}

/**
* mpage_writepages - walk the list of dirty pages of the given address space & writepage() all of them
* @mapping: address space structure to write
* @wbc: subtract the number of written pages from *@wbc->nr_to_write
* @get_block: the filesystem's block mapper function.
*
* This is a library function, which implements the writepages()
* address_space_operation.
*/
int
mpage_writepages(struct address_space *mapping,
  struct writeback_control *wbc, get_block_t get_block)
{
struct mpage_data mpd = {
  .get_block = get_block,
};
struct folio *folio = NULL;
struct blk_plug plug;
int error;

blk_start_plug(&plug);
while ((folio = writeback_iter(mapping, wbc, folio, &error)))
  error = mpage_write_folio(wbc, folio, &mpd);
if (mpd.bio)
  mpage_bio_submit_write(mpd.bio);
blk_finish_plug(&plug);
return error;
}
EXPORT_SYMBOL(mpage_writepages);

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