gistfile1.txt

### From: http://git.itanic.dy.fi/?p=linux-stable;a=commitdiff_plain;h=9b17038d06c8ca74ca510d1a546a5dfd67f86e25
###
# From: Jan Kara <jack@suse.cz>
# Date: Thu, 13 Jan 2011 23:45:48 +0000 (-0800)
# Subject: writeback: avoid livelocking WB_SYNC_ALL writeback
# X-Git-Tag: v2.6.37.1~145
# X-Git-Url: http://git.itanic.dy.fi/?p=linux-stable;a=commitdiff_plain;h=9b17038d06c8ca74ca510d1a546a5dfd67f86e25
#
# writeback: avoid livelocking WB_SYNC_ALL writeback
#
# commit b9543dac5bbc4aef0a598965b6b34f6259ab9a9b upstream.
#
# When wb_writeback() is called in WB_SYNC_ALL mode, work->nr_to_write is
# usually set to LONG_MAX.  The logic in wb_writeback() then calls
# __writeback_inodes_sb() with nr_to_write == MAX_WRITEBACK_PAGES and we
# easily end up with non-positive nr_to_write after the function returns, if
# the inode has more than MAX_WRITEBACK_PAGES dirty pages at the moment.
#
# When nr_to_write is <= 0 wb_writeback() decides we need another round of
# writeback but this is wrong in some cases!  For example when a single
# large file is continuously dirtied, we would never finish syncing it
# because each pass would be able to write MAX_WRITEBACK_PAGES and inode
# dirty timestamp never gets updated (as inode is never completely clean).
# Thus __writeback_inodes_sb() would write the redirtied inode again and
# again.
#
# Fix the issue by setting nr_to_write to LONG_MAX in WB_SYNC_ALL mode.  We
# do not need nr_to_write in WB_SYNC_ALL mode anyway since
# write_cache_pages() does livelock avoidance using page tagging in
# WB_SYNC_ALL mode.
#
# This makes wb_writeback() call __writeback_inodes_sb() only once on
# WB_SYNC_ALL.  The latter function won't livelock because it works on
#
# - a finite set of files by doing queue_io() once at the beginning
# - a finite set of pages by PAGECACHE_TAG_TOWRITE page tagging
#
# After this patch, program from http://lkml.org/lkml/2010/10/24/154 is no
# longer able to stall sync forever.
#
# [fengguang.wu@intel.com: fix locking comment]
# Signed-off-by: Jan Kara <jack@suse.cz>
# Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
# Cc: Johannes Weiner <hannes@cmpxchg.org>
# Cc: Dave Chinner <david@fromorbit.com>
# Cc: Christoph Hellwig <hch@lst.de>
# Cc: Jan Engelhardt <jengelh@medozas.de>
# Cc: Jens Axboe <axboe@kernel.dk>
# Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
# Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
# Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
# ---
#
# diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
# index 7cc67e3..cdbf7ac 100644
# --- a/fs/fs-writeback.c
# +++ b/fs/fs-writeback.c
# @@ -629,6 +629,7 @@ static long wb_writeback(struct bdi_writeback *wb,
#   };
#   unsigned long oldest_jif;
#   long wrote = 0;
# + long write_chunk;
#   struct inode *inode;
#
#   if (wbc.for_kupdate) {
# @@ -641,6 +642,24 @@ static long wb_writeback(struct bdi_writeback *wb,
#       wbc.range_end = LLONG_MAX;
#   }
#
# + /*
# +  * WB_SYNC_ALL mode does livelock avoidance by syncing dirty
# +  * inodes/pages in one big loop. Setting wbc.nr_to_write=LONG_MAX
# +  * here avoids calling into writeback_inodes_wb() more than once.
# +  *
# +  * The intended call sequence for WB_SYNC_ALL writeback is:
# +  *
# +  *      wb_writeback()
# +  *          __writeback_inodes_sb()     <== called only once
# +  *              write_cache_pages()     <== called once for each inode
# +  *                   (quickly) tag currently dirty pages
# +  *                   (maybe slowly) sync all tagged pages
# +  */
# + if (wbc.sync_mode == WB_SYNC_NONE)
# +     write_chunk = MAX_WRITEBACK_PAGES;
# + else
# +     write_chunk = LONG_MAX;
# +
#   wbc.wb_start = jiffies; /* livelock avoidance */
#   for (;;) {
#       /*
# @@ -667,7 +686,7 @@ static long wb_writeback(struct bdi_writeback *wb,
#           break;
#
#       wbc.more_io = 0;
# -     wbc.nr_to_write = MAX_WRITEBACK_PAGES;
# +     wbc.nr_to_write = write_chunk;
#       wbc.pages_skipped = 0;
#
#       trace_wbc_writeback_start(&wbc, wb->bdi);
# @@ -677,8 +696,8 @@ static long wb_writeback(struct bdi_writeback *wb,
#           writeback_inodes_wb(wb, &wbc);
#       trace_wbc_writeback_written(&wbc, wb->bdi);
#
# -     work->nr_pages -= MAX_WRITEBACK_PAGES - wbc.nr_to_write;
# -     wrote += MAX_WRITEBACK_PAGES - wbc.nr_to_write;
# +     work->nr_pages -= write_chunk - wbc.nr_to_write;
# +     wrote += write_chunk - wbc.nr_to_write;
#
#       /*
#        * If we consumed everything, see if we have more
# @@ -693,7 +712,7 @@ static long wb_writeback(struct bdi_writeback *wb,
#       /*
#        * Did we write something? Try for more
#        */
# -     if (wbc.nr_to_write < MAX_WRITEBACK_PAGES)
# +     if (wbc.nr_to_write < write_chunk)
#           continue;
#       /*
#        * Nothing written. Wait for some inode to