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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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