[Devel] [PATCH vz7] fs: avoid writeback busy-loop if redirty

[Maxim Patlasov]

Andrey, can you please review this one-liner below. I think it can
be responsible for inexplicable delays and CPU peaks in bugs like
https://jira.sw.ru/browse/PSBM-55919. Thanks, Maxim

wb_writeback() bails out only if either nr_pages comes to zero or
"progress" (as reported by writeback_sb_inodes or __writeback_inodes_wb)
is zero.

If a page is temporarily unavailable (is under fuse-writeback),
fuse_writepages_fill calls redirty_page_for_writepage, unlock the page
and returns OK. This OK results in progress=1. So wb_writeback
loops over writeback_sb_inodes/__writeback_inodes_wb again and again
until all nr_pages are exhausted.

The patch ensures that progress=0 if fuse_writepages_fill did nothing.

Signed-off-by: Maxim Patlasov <mpatlasov at virtuozzo.com>
---
 fs/fs-writeback.c |    3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 5c9a082..483bbb9 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -774,6 +774,9 @@ static long writeback_sb_inodes(struct super_block *sb,
 			if (work->nr_pages <= 0)
 				break;
 		}
+
+		BUG_ON(wbc.pages_skipped > write_chunk - wbc.nr_to_write);
+		wrote -= wbc.pages_skipped;
 	}
 	return wrote;
 }