On 9/19/18 2:15 AM, Ming Lei wrote:
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?
I don't think that (1) is correct, especially if N is not a power of two. In the skd driver I addressed this problem by using kmem_cache_create() and kmem_cache_alloc() instead of kmalloc(). kmem_cache_create() allows to specify the alignment explicitly.
Bart.
