linux-kernel - Re: [PATCH] mm: fix list corruptions on shmem shrinklist

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Date:   Fri, 4 Aug 2017 11:06:03 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>, "# .39.x" <stable@...nel.org>,
        Hugh Dickins <hughd@...gle.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH] mm: fix list corruptions on shmem shrinklist

On Thu, Aug 3, 2017 at 4:25 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Thu, Aug 3, 2017 at 4:11 PM, Andrew Morton <akpm@...ux-foundation.org> wrote:
>>
>> Where is this INIT_LIST_HEAD()?
>
> I think it's this one:
>
>         list_del_init(&info->shrinklist);
>
> in shmem_unused_huge_shrink().


Yes, this is correct. Sorry about confusion.


>
>> I'm not sure I'm understanding this.  AFAICT all the list operations to
>> which you refer are synchronized under spin_lock(&sbinfo->shrinklist_lock)?
>
> No, notice how shmem_unused_huge_shrink() does the
>
>         list_move(&info->shrinklist, &to_remove);
>
> and
>
>         list_move(&info->shrinklist, &list);
>
> to move to (two different) private lists under the shrinklist_lock,
> but once it is on that private "list/to_remove" list, it is then
> accessed outside the locked region.
>
> Honestly, I don't love this situation, or the patch, but I think the
> patch is likely the right thing to do.
>

Me neither. This is probably the quickest fix we could have,
other possible changes might need much more work.

Thanks.