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    SubjectRe: [BUGFIX][PATCH] memcg: fix memory migration of shmem swapcache
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    Hi,

    On Wed, Jan 5, 2011 at 1:00 PM, Daisuke Nishimura
    <nishimura@mxp.nes.nec.co.jp> wrote:
    > Hi.
    >
    > This is a fix for a problem which has bothered me for a month.
    >
    > ===
    > From: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
    >
    > In current implimentation, mem_cgroup_end_migration() decides whether the page
    > migration has succeeded or not by checking "oldpage->mapping".
    >
    > But if we are tring to migrate a shmem swapcache, the page->mapping of it is
    > NULL from the begining, so the check would be invalid.
    > As a result, mem_cgroup_end_migration() assumes the migration has succeeded
    > even if it's not, so "newpage" would be freed while it's not uncharged.
    >
    > This patch fixes it by passing mem_cgroup_end_migration() the result of the
    > page migration.
    >
    > Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
    Reviewed-by: Minchan Kim <minchan.kim@gmail.com>

    Nice catch. I don't oppose the patch.
    But as looking the code in unmap_and_move, I feel part of mem cgroup
    migrate is rather awkward.

    int unmap_and_move()
    {
    charge = mem_cgroup_prepare_migration(xxx);
    ..
    BUG_ON(charge); <-- BUG if it is charged?
    ..
    uncharge:
    if (!charge) <-- why do we have to uncharge !charge?
    mem_group_end_migration(xxx);
    ..
    }

    'charge' local variable isn't good. How about changing "uncharge" or whatever?
    Of course, It would be another patch.
    If you don't mind, I will send the patch or you may send the patch.

    Thanks,

    --
    Kind regards,
    Minchan Kim


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