linux-kernel - Re: [patch 13/13] mm: memcontrol: rewrite uncharge API

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Date:	Fri, 18 Jul 2014 17:12:54 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Michal Hocko <mhocko@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>, Tejun Heo <tj@...nel.org>,
	Vladimir Davydov <vdavydov@...allels.com>, linux-mm@...ck.org,
	cgroups@...r.kernel.org,
	Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [patch 13/13] mm: memcontrol: rewrite uncharge API

On Fri, Jul 18, 2014 at 4:45 PM, Johannes Weiner <hannes@...xchg.org> wrote:

> I assumed the source page would always be new, according to this part
> in fuse_try_move_page():
>
>         /*
>          * This is a new and locked page, it shouldn't be mapped or
>          * have any special flags on it
>          */
>         if (WARN_ON(page_mapped(oldpage)))
>                 goto out_fallback_unlock;
>         if (WARN_ON(page_has_private(oldpage)))
>                 goto out_fallback_unlock;
>         if (WARN_ON(PageDirty(oldpage) || PageWriteback(oldpage)))
>                 goto out_fallback_unlock;
>         if (WARN_ON(PageMlocked(oldpage)))
>                 goto out_fallback_unlock;
>
> However, it's in the page cache and I can't really convince myself
> that it's not also on the LRU.  Miklos, I have trouble pinpointing
> where oldpage is instantiated exactly and what state it might be in -
> can it already be on the LRU?

oldpage comes from ->readpages() (*NOT* ->readpage()), i.e. readahead.

AFAICS it is added to the LRU in read_cache_pages(), so it looks like
it is definitely on the LRU at that point.

Thanks,
Miklos
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