[PATCH 6.4 715/800] writeback: account the number of pages written back

Greg Kroah-Hartman
2023-07-16T20:26:19Z

From: Matthew Wilcox (Oracle) <willy@infradead.org>

[ Upstream commit 8344a3d44be3d18671e18c4ba23bb03dd21e14ad ]

nr_to_write is a count of pages, so we need to decrease it by the number
of pages in the folio we just wrote, not by 1. Most callers specify
either LONG_MAX or 1, so are unaffected, but writeback_sb_inodes() might
end up writing 512x as many pages as it asked for.

Dave added:

: XFS is the only filesystem this would affect, right? AFAIA, nothing
: else enables large folios and uses writeback through
: write_cache_pages() at this point...
:
: In which case, I'd be surprised if much difference, if any, gets
: noticed by anyone.

Link: https://lkml.kernel.org/r/20230628185548.981888-1-willy@infradead.org
Fixes: 793917d997df ("mm/readahead: Add large folio readahead")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Jan Kara <jack@suse.cz>
Cc: Dave Chinner <david@fromorbit.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
mm/page-writeback.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index db79439990073..6faa09f1783b3 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2434,6 +2434,7 @@ int write_cache_pages(struct address_space *mapping,

for (i = 0; i < nr_folios; i++) {
struct folio *folio = fbatch.folios[i];
+ unsigned long nr;

done_index = folio->index;

@@ -2471,6 +2472,7 @@ int write_cache_pages(struct address_space *mapping,

trace_wbc_writepage(wbc, inode_to_bdi(mapping->host));
error = writepage(folio, wbc, data);
+ nr = folio_nr_pages(folio);
if (unlikely(error)) {
/*
* Handle errors according to the type of
@@ -2489,8 +2491,7 @@ int write_cache_pages(struct address_space *mapping,
error = 0;
} else if (wbc->sync_mode != WB_SYNC_ALL) {
ret = error;
- done_index = folio->index +
- folio_nr_pages(folio);
+ done_index = folio->index + nr;
done = 1;
break;
}
@@ -2504,7 +2505,8 @@ int write_cache_pages(struct address_space *mapping,
* keep going until we have written all the pages
* we tagged for writeback prior to entering this loop.
*/
- if (--wbc->nr_to_write <= 0 &&
+ wbc->nr_to_write -= nr;
+ if (wbc->nr_to_write <= 0 &&
wbc->sync_mode == WB_SYNC_NONE) {
done = 1;
break;
--
2.39.2