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    Subject[PATCH 3.16.y-ckt 036/165] md/raid1: fix read balance when a drive is write-mostly.
    Date
    3.16.7-ckt9 -stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: =?UTF-8?q?Tom=C3=A1=C5=A1=20Hodek?= <tomas.hodek@volny.cz>

    commit d1901ef099c38afd11add4cfb3312c02ef21ec4a upstream.

    When a drive is marked write-mostly it should only be the
    target of reads if there is no other option.

    This behaviour was broken by

    commit 9dedf60313fa4dddfd5b9b226a0ef12a512bf9dc
    md/raid1: read balance chooses idlest disk for SSD

    which causes a write-mostly device to be *preferred* is some cases.

    Restore correct behaviour by checking and setting
    best_dist_disk and best_pending_disk rather than best_disk.

    We only need to test one of these as they are both changed
    from -1 or >=0 at the same time.

    As we leave min_pending and best_dist unchanged, any non-write-mostly
    device will appear better than the write-mostly device.

    Reported-by: Tomáš Hodek <tomas.hodek@volny.cz>
    Reported-by: Dark Penguin <darkpenguin@yandex.ru>
    Signed-off-by: NeilBrown <neilb@suse.de>
    Link: http://marc.info/?l=linux-raid&m=135982797322422
    Fixes: 9dedf60313fa4dddfd5b9b226a0ef12a512bf9dc
    Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
    ---
    drivers/md/raid1.c | 5 +++--
    1 file changed, 3 insertions(+), 2 deletions(-)

    diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
    index 55de4f6f7eaf..b96ee9d78aa3 100644
    --- a/drivers/md/raid1.c
    +++ b/drivers/md/raid1.c
    @@ -561,7 +561,7 @@ static int read_balance(struct r1conf *conf, struct r1bio *r1_bio, int *max_sect
    if (test_bit(WriteMostly, &rdev->flags)) {
    /* Don't balance among write-mostly, just
    * use the first as a last resort */
    - if (best_disk < 0) {
    + if (best_dist_disk < 0) {
    if (is_badblock(rdev, this_sector, sectors,
    &first_bad, &bad_sectors)) {
    if (first_bad < this_sector)
    @@ -570,7 +570,8 @@ static int read_balance(struct r1conf *conf, struct r1bio *r1_bio, int *max_sect
    best_good_sectors = first_bad - this_sector;
    } else
    best_good_sectors = sectors;
    - best_disk = disk;
    + best_dist_disk = disk;
    + best_pending_disk = disk;
    }
    continue;
    }

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