Re: [RFC][PATCH 2/5] memcg : handle swap cache

[KAMEZAWA Hiroyuki]

On Wed, 5 Nov 2008 09:07:49 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:

> On Tue, 4 Nov 2008 19:28:22 +0900
> Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx> wrote:
> 
> > On Tue, 4 Nov 2008 18:04:29 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
> > > On Tue, 4 Nov 2008 17:42:01 +0900
> > > Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx> wrote:
> > > 
> > > > > +#ifdef CONFIG_SWAP
> > > > > +int mem_cgroup_cache_charge_swapin(struct page *page,
> > > > > +			struct mm_struct *mm, gfp_t mask)
> > > > > +{
> > > > > +	int ret = 0;
> > > > > +
> > > > > +	if (mem_cgroup_subsys.disabled)
> > > > > +		return 0;
> > > > > +	if (unlikely(!mm))
> > > > > +		mm = &init_mm;
> > > > > +
> > > > > +	ret = mem_cgroup_charge_common(page, mm, mask,
> > > > > +			MEM_CGROUP_CHARGE_TYPE_SHMEM, NULL);
> > > > > +	/*
> > > > > +	 * The page may be dropped from SwapCache because we don't have
> > > > > +	 * lock_page().This may cause charge-after-uncharge trouble.
> > > > > +	 * Fix it up here. (the caller have refcnt to this page and
> > > > > +	 * page itself is guaranteed not to be freed.)
> > > > > +	 */
> > > > > +	if (ret && !PageSwapCache(page))
> > > > > +		mem_cgroup_uncharge_swapcache(page);
> > > > > +
> > > > Hmm.. after [5/5], mem_cgroup_cache_charge_swapin has 'locked' parameter,
> > > > calls lock_page(if !locked), and checks PageSwapCache under page lock.
> > > > 
> > > > Why not doing it in this patch?
> > > > 
> > > 
> > > My intention is to guard swap_cgroup by lock_page() against SwapCache.
> > > In Mem+Swap controller. we get "memcg" from information in page->private.
> > > I think we need lock_page(), there. 
> > > 
> > > But here, we don't refer page->private information. 
> > > I think we don't need lock_page() because there is no inofrmation we depends on.
> > > 
> > I just thought it would be simpler to check PageSwapCache after holding
> > page lock rather than to handle the case that the page might be removed from
> > swap cache.
> > 
> > And to be honest, I can't understand the "charge-after-uncharge trouble".
> > Could you explain more?
> > 

I'll add lock_page() here to make this simple.

Thanks,
-Kame


> Maybe typical case is following.
> __delete_from_swapcache can happen while the page is unlocked.
> ==
>                                                   some other thread.
>    page = shmem_swapin()
>    	swapin_readahead();
>    # page is SwapCache here.
>    # but this page is not locked.
>                                                   ___delete_from_swapcache(page)
>    # This is not SwapCache.                                 => uncharge swapcache.
>    mem_cgroup_charge_cache_swapin();
>    {
>        charge();  # charged this page but we don't know this is still swapcache.
>        if (!PageSwapCache(page)) {
> 		# Oh we should unroll this.
>        }
>    }
> =
> 
> Thanks,
> -Kame
> 
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