Messages in this thread | | | From | Ming Lei <> | Date | Wed, 19 Sep 2018 17:15:43 +0800 | Subject | block: DMA alignment of IO buffer allocated from slab |
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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.
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?
3) If slab can't guarantee to return 512-aligned buffer, how to fix this data corruption issue?
Thanks, Ming Lei
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