linux-kernel - Re: [PATCH] tmpfs: fix shmem_getpage_gfp VM_BUG

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Message-ID: <50A58B6E.8090609@gmail.com>
Date:	Fri, 16 Nov 2012 08:40:14 +0800
From:	Jaegeuk Hanse <jaegeuk.hanse@...il.com>
To:	Hugh Dickins <hughd@...gle.com>
CC:	Dave Jones <davej@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tmpfs: fix shmem_getpage_gfp VM_BUG_ON

On 11/16/2012 03:56 AM, Hugh Dickins wrote:
> Offtopic...
>
> On Thu, 15 Nov 2012, Jaegeuk Hanse wrote:
>> Another question. Why the function shmem_fallocate which you add to kernel
>> need call shmem_getpage?
> Because shmem_getpage(_gfp) is where shmem's
> page lookup and allocation complexities are handled.
>
> I assume the question behind your question is: why does shmem actually
> allocate pages for its fallocate, instead of just reserving the space?

Yeah, this is what I want to know.

>
> I did play with just reserving the space, with more special entries in
> the radix_tree to note the reservations made.  It should be doable for
> the vm_enough_memory and sbinfo->used_blocks reservations.
>
> What absolutely deterred me from taking that path was the mem_cgroup
> case: shmem and swap and memcg are not easy to get working right together,
> and nobody would thank me for complicating memcg just for shmem_fallocate.
>
> By allocating pages, the pre-existing memcg code just works; if we used
> reservations instead, we would have to track their memcg charges in some
> additional new way.  I see no justification for that complication.

Oh, I see, thanks Hugh. :-)

> Hugh

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