linux-kernel - Re: [PATCH -next] ashmem: Fix ashmem

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Message-ID: <CAG6tG3zaU=gkJZzNezOX=s5WWDrc2x7SHiDy3UxVzDvKkw2foA@mail.gmail.com>
Date:	Thu, 16 May 2013 09:44:49 -0400
From:	Robert Love <rlove@...gle.com>
To:	Raul Xiong <raulxiong@...il.com>
Cc:	Neil Zhang <glacier1980@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Shankar Brahadeeswaran <shankoo77@...il.com>,
	Dan Carpenter <dan.carpenter@...cle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Bjorn Bringert <bringert@...gle.com>,
	devel <devel@...verdev.osuosl.org>,
	Hugh Dickins <hughd@...gle.com>,
	Anjana V Kumar <anjanavk12@...il.com>,
	Dad <akpm@...ux-foundation.org>,
	linux-next <linux-next@...r.kernel.org>
Subject: Re: [PATCH -next] ashmem: Fix ashmem_shrink deadlock.

On Thu, May 16, 2013 at 4:15 AM, Raul Xiong <raulxiong@...il.com> wrote:
> The issue happens in such sequence:
> ashmem_mmap acquired ashmem_mutex --> ashmem_mutex:shmem_file_setup
> called kmem_cache_alloc --> shrink due to low memory --> ashmem_shrink
> tries to acquire the same ashmem_mutex -- it blocks here.
>
> I think this reports the bug clearly. Please have a look.

There is no debate about the nature of the bug. Only the fix.

My mutex_trylock patch fixes the problem. I prefer that solution.

Andrew's suggestion of GFP_ATOMIC won't work as we'd have to propagate
that down into shmem and elsewhere.

Using PF_MEMALLOC will work. You'd want to define something like:

static int set_memalloc(void)
{
        if (current->flags & PF_MEMALLOC)
                return 0;
        current->flags |= PF_MEMALLOC;
        return 1;
}

static void clear_memalloc(int memalloc)
{
        if (memalloc)
                current->flags &= ~PF_MEMALLOC;
}

and then set/clear PF_MEMALLOC around every memory allocation and
function that descends into a memory allocation. As said I prefer my
solution but if someone wants to put together a patch with this
approach, fine by me.

         Robert
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