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SubjectRe: [patch] mm: memcg: close race between charge and putback
On Thu, 8 Sep 2011 10:54:04 +0200
Johannes Weiner <jweiner@redhat.com> wrote:

> On Thu, Sep 08, 2011 at 05:30:42PM +0900, KAMEZAWA Hiroyuki wrote:
> > On Thu, 8 Sep 2011 09:40:22 +0200
> > Johannes Weiner <jweiner@redhat.com> wrote:
> >
> > > There is a potential race between a thread charging a page and another
> > > thread putting it back to the LRU list:
> > >
> > > charge: putback:
> > > SetPageCgroupUsed SetPageLRU
> > > PageLRU && add to memcg LRU PageCgroupUsed && add to memcg LRU
> > >
> >
> > I assumed that all pages are charged before added to LRU.
> > (i.e. event happens in charge->lru_lock->putback order.)
> >
> > But hmm, this assumption may be bad for maintainance.
> > Do you find a code which adds pages to LRU before charge ?
> >
> > Hmm, if there are codes which recharge the page to other memcg,
> > it will cause bug and my assumption may be harmful.
>
> Swap slots are read optimistically into swapcache and put to the LRU,
> then charged upon fault.

Yes, then swap charge removes page from LRU before charge.
IIUC, it needed to do so because page->mem_cgroup may be replaced.

> Fuse apparently recharges uncharged LRU pages.
Yes and No. IIUC, it was like page migraion and remove old page
and add new page to radix-tree.

> That's why we have the lrucare stuff in the first place, no?
You're right.

> Or did I misunderstand your question?
>

I just wondered whether you find a new one or possible user.

Thanks,
-Kame




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