Messages in this thread | | | Date | Wed, 5 Jan 2011 16:18:27 +0900 | Subject | Re: [BUGFIX][PATCH] memcg: fix memory migration of shmem swapcache | From | Minchan Kim <> |
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On Wed, Jan 5, 2011 at 3:47 PM, Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote: > On Wed, 5 Jan 2011 13:48:50 +0900 > Minchan Kim <minchan.kim@gmail.com> wrote: > >> 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. > Thank you for your review. > >> 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? > hmm, I agree that current code seems a bit confusing, but I can't think of > better name to imply the result of 'charge'. > > And considering more, I can't understand why we need to check "if (!charge)" > before mem_cgroup_end_migration() becase it must be always true and, IMHO, > mem_cgroup_end_migration() should do all necesarry checks to avoid double uncharge. > So, I think this local variable can be removed completely.
Agree.
-- Kind regards, Minchan Kim -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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