Messages in this thread | | | Date | Thu, 6 Jan 2011 09:52:11 +0900 | From | KAMEZAWA Hiroyuki <> | Subject | Re: [BUGFIX][PATCH] memcg: fix memory migration of shmem swapcache |
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On Wed, 5 Jan 2011 15:47:48 +0900 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. >
Nice catch.
> > 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.
ok, please remove it. Before this commit, http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=01b1ae63c2270cbacfd43fea94578c17950eb548;hp=bced0520fe462bb94021dcabd32e99630c171be2
"mem" is not passed as argument and this was the reason for the vairable "charge".
We can check "charge is in moving" by checking "mem == NULL".
> So, I think this local variable can be removed completely. > > rc = mem_cgroup_prepare_migration(..); > if (rc == -ENOMEM) > goto unlock; > BUG_ON(rc); > .. > uncharge: > mem_cgroup_end_migration(..); > > KAMEZAWA-san, what do you think ? >
seems ok.
Thanks, -Kame
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