On Wed, Sep 19, 2018 at 6:28 PM Ming Lei <ming.lei@xxxxxxxxxx> wrote: > > On Wed, Sep 19, 2018 at 01:15:00PM +0200, Vitaly Kuznetsov wrote: > > Ming Lei <ming.lei@xxxxxxxxxx> writes: > > > > > Hi Vitaly, > > > > > > On Wed, Sep 19, 2018 at 11:41:07AM +0200, Vitaly Kuznetsov wrote: > > >> 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? > > > > > > 512-byte alignment is also 256-byte aligned, and the sector size is 512 byte. > > > > > > > Yes, but it doesn't work the other way around, e.g. what if some device > > has e.g. PAGE_SIZE alignment requirement (this would likely imply that > > it's sector size is also not 512 I guess)? > > Yeah, that can be true if one controller has 4k-byte sector size, also > its DMA alignment is 4K. But there shouldn't be cases in which the two > doesn't match. > > > > > > > > > From the Red Hat BZ, looks I understand this issue is only triggered when > > > KASAN is enabled, or you have figured out how to reproduce it without > > > KASAN involved? > > > > Yes, any SLUB debug triggers it (e.g. build your kernel with > > SLUB_DEBUG_ON or slub_debug= options (Red zoning, User tracking, ... - > > everything will trigger it) > > That means the slab always return 512-byte aligned buffer if the buffer > size is 512byte in case of no any slab debug options enabled. > > The question is that if it is one reliable rule in slab. If yes, any > slab debug option does violate the rule. Once slub debug (i.e. red zone) is on, it will append extra bytes to the object, so the object may look like: ----------------------------------------------------------------- | object | red zone | FP | owner track | red zone | ------------------------------------------------------------------ This is how slub debug is designed and how it works. CC to Chris Lameter who is the maintainer of SLUB. Regards, Yang > > The same is true for 4k alignment and 4k sector size. > > I think we need our MM guys to clarify this point. > > > Thanks, > Ming >
