Re: [PATCH] xfs: allocate sector sized IO buffer via page_frag_alloc

[Vlastimil Babka <vbabka@suse.cz>]

On 2/26/19 5:19 PM, Matthew Wilcox wrote:
> On Tue, Feb 26, 2019 at 08:14:33AM -0800, Darrick J. Wong wrote:
>> On Tue, Feb 26, 2019 at 06:04:40AM -0800, Matthew Wilcox wrote:
>> Wait a minute, are you all saying that /directio/ is broken on XFS too??
>> XFS doesn't use blockdev_direct_IO anymore.
>>
>> I thought we were talking about alignment of XFS metadata buffers
>> (xfs_buf.c), which is a very different topic.
>>
>> As I understand the problem, in non-debug mode the slab caches give
>> xfs_buf chunks of memory that are aligned well enough to work, but in
>> debug mode the slabs allocate slightly more bytes to carry debug
>> information which pushes the returned address up slightly, thus breaking
>> the alignment requirements.
>>
>> So why can't we just move the debug info to the end of the object?  If
>> our 512 byte allocation turns into a (512 + a few more) bytes we'll end
>> up using 1024 bytes on the allocation regardless, so it shouldn't matter
>> to put the debug info at offset 512.  If the reason is fear that kernel
>> code will scribble off the end of the object, then return (*obj + 512).
>> Maybe you all have already covered this, though?
> 
> I don't know _what_ Ming Lei is saying.  I thought the problem was
> with slab redzones, which need to be before and after each object,
> but apparently the problem is with KASAN as well.

That's what I thought as well. But if we can solve it for caches created
by kmem_cache_create(..., align, ...) then IMHO we could guarantee
natural alignment for power-of-two kmalloc caches as well.