linux-kernel - Re: [RESEND PATCH v5 1/4] zram: Fix deadlock bug in partial write

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Message-ID: <CAOJsxLG=3sWdFkvb++f2ywhs288ozoMXZd_g2WOAgxTqxayY_Q@mail.gmail.com>
Date:	Mon, 28 Jan 2013 15:26:26 +0200
From:	Pekka Enberg <penberg@...nel.org>
To:	Jerome Marchand <jmarchan@...hat.com>
Cc:	Minchan Kim <minchan@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Nitin Gupta <ngupta@...are.org>,
	Konrad Rzeszutek Wilk <konrad@...nok.org>,
	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	stable@...r.kernel.org
Subject: Re: [RESEND PATCH v5 1/4] zram: Fix deadlock bug in partial write

On Mon, Jan 28, 2013 at 1:24 PM, Jerome Marchand <jmarchan@...hat.com> wrote:
> On 01/28/2013 08:16 AM, Pekka Enberg wrote:
>> On Mon, Jan 28, 2013 at 2:38 AM, Minchan Kim <minchan@...nel.org> wrote:
>>> Now zram allocates new page with GFP_KERNEL in zram I/O path
>>> if IO is partial. Unfortunately, It may cuase deadlock with
>>
>> s/cuase/cause/g
>>
>>> reclaim path so this patch solves the problem.
>>
>> It'd be nice to know about the problem in more detail. I'm also
>> curious on why you decided on GFP_ATOMIC for the read path and
>> GFP_NOIO in the write path.
>
> This is because we're holding a kmap_atomic page in the read path.

Okay, so that's about partial *reads* and not even mentioned in the
changelog, no?

AFAICT, you could rearrange the code in zram_bvec_read() as follows:

        if (is_partial_io(bvec))
                /* Use  a temporary buffer to decompress the page */
                uncmem = kmalloc(PAGE_SIZE, GFP_KERNEL);
        else {
                uncmem = user_mem = kmap_atomic(page);
        }

and avoid the GFP_ATOMIC allocation.
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