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

[Joonsoo Kim]

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@xxxxxxx>

Thanks.
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