Ming Lei <tom.leiming@xxxxxxxxx> 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
