linux-kernel - Re: [PATCH] fix crash when using XFS on loopback

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Date:	Wed, 08 Jan 2014 22:42:03 +0100
From:	Helge Deller <deller@....de>
To:	Pekka Enberg <penberg@...nel.org>
CC:	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Mikulas Patocka <mpatocka@...hat.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Christoph Lameter <cl@...ux.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-parisc@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] fix crash when using XFS on loopback

On 01/08/2014 10:37 PM, Pekka Enberg wrote:
> On Wed, Jan 8, 2014 at 11:05 PM, Helge Deller <deller@....de> wrote:
>> On 01/07/2014 02:41 AM, Joonsoo Kim wrote:
>>> On Mon, Jan 06, 2014 at 12:54:22PM -0500, Mikulas Patocka wrote:
>>>> Hi
>>>>
>>>> On Mon, 6 Jan 2014, Joonsoo Kim wrote:
>>>>
>>>>> Hello,
>>>>>
>>>>> I'm surprised that this VM_BUG_ON() has not been triggered until now. It
>>>>> was
>>>>> introduced in 2007 by commit (b5fab14). Maybe there is no person who
>>>>> test
>>>>> with CONFIG_DEBUG_VM.
>>>> Last time I tried it, PS-RISC didn't work with CONFIG_DEBUG_VM at all.
>>>>
>>>>> There is one more bug report same as this.
>>>>> * possible regression on 3.13 when calling flush_dcache_page
>>>>>     (lkml.org/lkml/2013/12/12/255)
>>>> That link doesn't show anything.
>>>>
>>>>> As mentioned in the description of commit (b5fab14), slab object may not
>>>>> be
>>>>> properly aligned and use of page oriented function to this object can be
>>>>> dangerous. I searched the XFS code and found that they only try to
>>>>> allocate
>>>>> multiple of 512 bytes, so there is no problem for now. But, IMHO, it is
>>>>> better
>>>>> not to use slab objects for this purpose.
>>>> If slab debugging is enabled, kmalloc memory is not aligned.
>>>>
>>>> In XFS in xfs_buf_allocate_memory they test if the kmalloc memory crosses
>>>> page boundary - if it does, they free the kmalloc memory and allocate a
>>>> full page. Maybe this approach could still run into problems with some
>>>> bus-master adapters that assume alignment in hardware...
>>>>
>>>>
>>>> dm-bufio also does I/O to slab-allocated buffers, but it allocates the
>>>> object from slab (not kmalloc) with proper alignment.
>>> Hello,
>>>
>>> Okay. I see.
>>> Thanks for good explanation.
>>>
>>>>> And I rapidly searched every callsites of page_mapping() and, IMHO, this
>>>>> patch would work correctly. But possibly reverting original commit is
>>>>> better solution.
>>>> Reverting the original commit wouldn't fix that VM_BUG_ON.
>>> Initially, I thought that VM_BUG_ON() isn't wrong and it was better to
>>> remove
>>> the callsites where do I/O with slab-allocated buffers, because doing I/O
>>> with slab-allocated buffers needs a great care. So I didn't fully agreed
>>> with
>>> your patch and recommended to revert original commit yesterday. After
>>> reverting
>>> that, I would attempt to remove the callsites.
>>>
>>> But, now, I change my thought, because of your explanation. There are
>>> already
>>> some users to do I/O with slab-allocated buffers and they already did it
>>> with
>>> some cares, so I guess that admitting this usage is more beneficial than
>>> forbidding it.
>>>
>>> Reviewed-by: Joonsoo Kim <iamjoonsoo.kim@....com>
>>
>> I can queue up this patch in my next pull-request for the parisc-tree which
>> I plan to
>> send tomorrow, unless people want this patch to go via mm-tree or
>> similiar...
>> Please let me know.
> The patch looks good to me but it probably should go through Andrew's tree.
>
> Acked-by: Pekka Enberg <penberg@...nel.org>

Absolutely fine with me. Andrew, can you please pick it up for 3.13 ?
Thanks,
Helge

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