Re: block: DMA alignment of IO buffer allocated from slab

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Ming Lei <tom.leiming@gmail.com> writes:

> Hi Guys,
>
> Some storage controllers have DMA alignment limit, which is often set via
> blk_queue_dma_alignment(), such as 512-byte alignment for IO buffer.

While mostly drivers use 512-byte alignment it is not a rule of thumb,
'git grep' tell me we have:
ide-cd.c with 32-byte alignment
ps3disk.c and rsxx/dev.c with variable alignment.

What if our block configuration consists of several devices (in raid
array, for example) with different requirements, e.g. one requiring
512-byte alignment and the other requiring 256?

>
> Block layer now only checks if this limit is respected for buffer of
> pass-through request,
> see blk_rq_map_user_iov(), bio_map_user_iov().
>
> The userspace buffer for direct IO is checked in dio path, see
> do_blockdev_direct_IO().
> IO buffer from page cache should be fine wrt. this limit too.
>
> However, some file systems, such as XFS, may allocate single sector IO buffer
> via slab. Usually I guess kmalloc-512 should be fine to return
> 512-aligned buffer.
> But once KASAN or other slab debug options are enabled, looks this
> isn't true any
> more, kmalloc-512 may not return 512-aligned buffer. Then data corruption
> can be observed because the IO buffer from fs layer doesn't respect the DMA
> alignment limit any more.
>
> Follows several related questions:
>
> 1) does kmalloc-N slab guarantee to return N-byte aligned buffer?  If
> yes, is it a stable rule?
>
> 2) If it is a rule for kmalloc-N slab to return N-byte aligned buffer,
> seems KASAN violates this
> rule?

(as I was kinda involved in debugging): the issue was observed with SLUB
allocator KASAN is not to blame, everything wich requires aditional
metadata space will break this, see e.g. calculate_sizes() in slub.c

>
> 3) If slab can't guarantee to return 512-aligned buffer, how to fix
> this data corruption issue?

I'm no expert in block layer but in case of complex block device
configurations when bio submitter can't know all the requirements I see
no other choice than bouncing.

-- 
  Vitaly