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SubjectRe: [RFC][PATCH 2/5] memcg : handle swap cache
On Tue, 4 Nov 2008 17:42:01 +0900
Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> 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.

Thanks,
-Kame



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