From: Eric Sandeen <sandeen@redhat.com>
Date: Thu, 28 Oct 2010 01:30:13 +0000 (-0400)
Subject: ext4: implement writeback livelock avoidance using page tagging
X-Git-Tag: v2.6.37-rc1~76^2^2~12
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=5b41d92437f1ae19b3f3ffa3b16589fd5df50ac0;p=pandora-kernel.git

ext4: implement writeback livelock avoidance using page tagging

This is analogous to Jan Kara's commit,
f446daaea9d4a420d16c606f755f3689dcb2d0ce
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: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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