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

[Mikulas Patocka]

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.

> 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.

> Hello, Pekka and Christoph.
> Could you teach me which direction we have to go?
> 
> Thanks.

Mikulas
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