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Date: Wed, 8 Jan 2014 23:37:49 +0200 From: Pekka Enberg <penberg@...nel.org> To: Helge Deller <deller@....de> 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 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> -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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