Re: [patch] mm: memcg: close race between charge and putback

[Johannes Weiner <jweiner@redhat.com>]

On Thu, Sep 08, 2011 at 06:19:01PM +0900, KAMEZAWA Hiroyuki wrote:
> 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.

But only from the memcg LRU.  It's still on the global per-zone LRU,
so reclaim could isolate/putback it during the charge.  And then

> > > > charge:                         putback:
> > > > SetPageCgroupUsed               SetPageLRU
> > > > PageLRU && add to memcg LRU     PageCgroupUsed && add to memcg LRU

applies.

And yes, it needs to fix up *pc->mem_cgroup's LRU statistics before
the pointer get's overwritten.