Commit 5b41d924 authored by Eric Sandeen's avatar Eric Sandeen Committed by Theodore Ts'o
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ext4: implement writeback livelock avoidance using page tagging 



This is analogous to Jan Kara's commit,
f446daae
mm: implement writeback livelock avoidance using page tagging

but since we forked write_cache_pages, we need to reimplement
it there (and in ext4_da_writepages, since range_cyclic handling
was moved to there)

If you start a large buffered IO to a file, and then set
fsync after it, you'll find that fsync does not complete
until the other IO stops.

If you continue re-dirtying the file (say, putting dd
with conv=notrunc in a loop), when fsync finally completes
(after all IO is done), it reports via tracing that
it has written many more pages than the file contains;
in other words it has synced and re-synced pages in
the file multiple times.

This then leads to problems with our writeback_index
update, since it advances it by pages written, and
essentially sets writeback_index off the end of the
file...

With the following patch, we only sync as much as was
dirty at the time of the sync.

Signed-off-by: default avatarEric Sandeen <sandeen@redhat.com>
Signed-off-by: default avatar"Theodore Ts'o" <tytso@mit.edu>
parent bbd08344

fs/ext4/inode.c

+15 −3
Original line number Diff line number Diff line
@@ -2809,16 +2809,21 @@ static int write_cache_pages_da(struct address_space *mapping, 
	pgoff_t index;

	pgoff_t end;		/* Inclusive */

	long nr_to_write = wbc->nr_to_write;

	int tag;



	pagevec_init(&pvec, 0);

	index = wbc->range_start >> PAGE_CACHE_SHIFT;

	end = wbc->range_end >> PAGE_CACHE_SHIFT;



	if (wbc->sync_mode == WB_SYNC_ALL)

		tag = PAGECACHE_TAG_TOWRITE;

	else

		tag = PAGECACHE_TAG_DIRTY;



	while (!done && (index <= end)) {

		int i;



		nr_pages = pagevec_lookup_tag(&pvec, mapping, &index,

			      PAGECACHE_TAG_DIRTY,

		nr_pages = pagevec_lookup_tag(&pvec, mapping, &index, tag,

			      min(end - index, (pgoff_t)PAGEVEC_SIZE-1) + 1);

		if (nr_pages == 0)

			break;
@@ -2923,6 +2928,7 @@ static int ext4_da_writepages(struct address_space *mapping, 
	long desired_nr_to_write, nr_to_writebump = 0;

	loff_t range_start = wbc->range_start;

	struct ext4_sb_info *sbi = EXT4_SB(mapping->host->i_sb);

	pgoff_t end;



	trace_ext4_da_writepages(inode, wbc);
@@ -2958,8 +2964,11 @@ static int ext4_da_writepages(struct address_space *mapping, 
		wbc->range_start = index << PAGE_CACHE_SHIFT;

		wbc->range_end  = LLONG_MAX;

		wbc->range_cyclic = 0;

	} else

		end = -1;

	} else {

		index = wbc->range_start >> PAGE_CACHE_SHIFT;

		end = wbc->range_end >> PAGE_CACHE_SHIFT;

	}



	/*

	 * This works around two forms of stupidity.  The first is in
@@ -3000,6 +3009,9 @@ static int ext4_da_writepages(struct address_space *mapping, 
	pages_skipped = wbc->pages_skipped;



retry:

	if (wbc->sync_mode == WB_SYNC_ALL)

		tag_pages_for_writeback(mapping, index, end);



	while (!ret && wbc->nr_to_write > 0) {



		/*

include/linux/writeback.h

+2 −0
Original line number Diff line number Diff line
@@ -143,6 +143,8 @@ typedef int (*writepage_t)(struct page *page, struct writeback_control *wbc, 


int generic_writepages(struct address_space *mapping,

		       struct writeback_control *wbc);

void tag_pages_for_writeback(struct address_space *mapping,

			     pgoff_t start, pgoff_t end);

int write_cache_pages(struct address_space *mapping,

		      struct writeback_control *wbc, writepage_t writepage,

		      void *data);

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