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SubjectRe: [BUGFIX take 3] [PATCH] write-back: fix nr_to_write counter
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[Take into account Dave Kleikamp's suggestion and fix commit message]

From: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Date: Mon, 2 Feb 2009 18:33:49 +0200
Subject: [PATCH] write-back: fix nr_to_write counter

Commit 05fe478dd04e02fa230c305ab9b5616669821dd3 introduced some
@wbc->nr_to_write breakage. Here is the change from the commit:

>--- a/mm/page-writeback.c
>+++ b/mm/page-writeback.c
>@@ -963,8 +963,10 @@ retry:
> }
> }
>
> if (--nr_to_write <= 0)
> done = 1;
> if (wbc->sync_mode == WB_SYNC_NONE) {
> if (--wbc->nr_to_write <= 0)
> done = 1;
> }
> if (wbc->nonblocking && bdi_write_congested(bdi)) {
> wbc->encountered_congestion = 1;
> done = 1
> }

It makes the following changes:
1. Decrement wbc->nr_to_write instead of nr_to_write
2. Decrement wbc->nr_to_write _only_ if wbc->sync_mode == WB_SYNC_NONE
3. If synced nr_to_write pages, stop only if if wbc->sync_mode ==
WB_SYNC_NONE, otherwise keep going.

However, according to the commit message, the intention was to
only make change 3. Change 1 is a bug. Change 2 does not seem to be
necessary, and it breaks UBIFS expectations, so if needed, it
should be done separately later. And change 2 does not seem to
be documented in the commit message.

This patch does the following:
1. Undo changes 1 and 2
2. Add a comment explaining change 3 (it very useful to have comments in
_code_, not only in the commit).

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Acked-by: Nick Piggin <npiggin@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
mm/page-writeback.c | 21 +++++++++++++++------
1 files changed, 15 insertions(+), 6 deletions(-)
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index b493db7..13a2b8e 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1051,13 +1051,22 @@ continue_unlock:
}
}

- if (wbc->sync_mode == WB_SYNC_NONE) {
- wbc->nr_to_write--;
- if (wbc->nr_to_write <= 0) {
- done = 1;
- break;
- }
+ if (nr_to_write > 0)
+ nr_to_write--;
+ else if (wbc->sync_mode == WB_SYNC_NONE) {
+ /*
+ * We stop writing back only if we are not
+ * doing integrity sync. In case of integrity
+ * sync we have to keep going because someone
+ * may be concurrently dirtying pages, and we
+ * might have synced a lot of newly appeared
+ * dirty pages, bud have not synced all of the
+ * old dirty pages.
+ */
+ done = 1;
+ break;
}
+
if (wbc->nonblocking && bdi_write_congested(bdi)) {
wbc->encountered_congestion = 1;
done = 1;
--
1.6.0.6

--
Best regards,
Artem Bityutskiy (Битюцкий Артём)

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