linux-kernel - Re: shmem_getpage_gfp VM_BUG

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Date:	Thu, 25 Oct 2012 14:27:41 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Ni zhan Chen <nizhan.chen@...il.com>
cc:	Dave Jones <davej@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: shmem_getpage_gfp VM_BUG_ON triggered. [3.7rc2]

On Thu, 25 Oct 2012, Ni zhan Chen wrote:
> On 10/25/2012 02:59 PM, Hugh Dickins wrote:
> > On Thu, 25 Oct 2012, Ni zhan Chen wrote:
> > > 
> > > I think it maybe caused by your commit [d189922862e03ce: shmem: fix
> > > negative
> > > rss in memcg memory.stat], one question:
> > Well, yes, I added the VM_BUG_ON in that commit.
> > 
> > > if function shmem_confirm_swap confirm the entry has already brought back
> > > from swap by a racing thread,
> > The reverse: true confirms that the swap entry has not been brought back
> > from swap by a racing thread; false indicates that there has been a race.
> > 
> > > then why call shmem_add_to_page_cache to add
> > > page from swapcache to pagecache again?
> > Adding it to pagecache again, after such a race, would set error to
> > -EEXIST (originating from radix_tree_insert); but we don't do that,
> > we add it to pagecache when it has not already been added.
> > 
> > Or that's the intention: but Dave seems to have found an unexpected
> > exception, despite us holding the page lock across all this.
> > 
> > (But if it weren't for the memcg and replace_page issues, I'd much
> > prefer to let shmem_add_to_page_cache discover the race as before.)
> > 
> > Hugh
> 
> Hi Hugh
> 
> Thanks for your response. You mean the -EEXIST originating from
> radix_tree_insert, in radix_tree_insert:
> if (slot != NULL)
>     return -EEXIST;
> But why slot should be NULL? if no race, the pagecache related radix tree
> entry should be RADIX_TREE_EXCEPTIONAL_ENTRY+swap_entry_t.val, where I miss?

I was describing what would happen in a case that should not exist,
that you had thought the common case.  In actuality, the entry should
not be NULL, it should be as you say there.

Hugh
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