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

[Helge Deller]

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@xxxxxxx>

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
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