linux-kernel - Re: [PATCH] proc: put check_mem_permission before __get_free_page in mem

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Message-ID: <BANLkTi=92cJNdvmjpKkYZk_Q2XvzXjnbCw@mail.gmail.com>
Date:	Tue, 26 Apr 2011 16:55:19 +0800
From:	Jovi Zhang <bookjovi@...il.com>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	David Rientjes <rientjes@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Stephen Wilson <wilsons@...rt.ca>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] proc: put check_mem_permission before __get_free_page in mem_read

On Tue, Apr 26, 2011 at 11:56 AM, Hugh Dickins <hughd@...gle.com> wrote:
> On Sun, 17 Apr 2011, bookjovi@...il.com wrote:
>> From: Jovi Zhang <bookjovi@...il.com>
>>
>> It should be better if put check_mem_permission before __get_free_page
>> in mem_read, to be same as function mem_write.
>>
>> Signed-off-by: Jovi Zhang <bookjovi@...il.com>
>
> Sorry to be contrary, but I disagree with this.  I'm all for consistency,
> but is there a particular reason why you think the mem_write ordering is
> right and mem_read wrong?
>
> My reason for preferring the current mem_read ordering is this:
>
> check_mem_permission gets a reference to the mm.  If we __get_free_page
> after check_mem_permission, imagine what happens if the system is out
> of memory, and the mm we're looking at is selected for killing by the
> OOM killer: while we wait in __get_free_page for more memory, no memory
> is freed from the selected mm because it cannot reach exit_mmap while
> we hold that reference.
>
> (I may be overstating the case: a little memory may be freed from the
> exiting task's stack, and kswapd should still be able to pick some pages
> off the mm.  But nonetheless, we would do better to let this mm go.)
>

Indeed, I missed that point. Thanks.
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