From mboxrd@z Thu Jan 1 00:00:00 1970 From: Helge Deller Subject: Re: [PATCH] fix crash when using XFS on loopback Date: Wed, 08 Jan 2014 22:05:24 +0100 Message-ID: <52CDBD94.9090808@gmx.de> References: <20140106073549.GA25771@lge.com> <20140107014159.GA26726@lge.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Cc: Andi Kleen , Christoph Lameter , Pekka Enberg , linux-kernel@vger.kernel.org, linux-parisc@vger.kernel.org To: Joonsoo Kim , Mikulas Patocka Return-path: In-Reply-To: <20140107014159.GA26726@lge.com> List-ID: List-Id: linux-parisc.vger.kernel.org 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 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. Helge