On Wed, Aug 28, 2019 at 12:46:08PM -0700, Matthew Wilcox wrote: > On Wed, Aug 28, 2019 at 06:45:07PM +0000, Christopher Lameter wrote: > > I still think implicit exceptions to alignments are a bad idea. Those need > > to be explicity specified and that is possible using kmem_cache_create(). > > I swear we covered this last time the topic came up, but XFS would need > to create special slab caches for each size between 512 and PAGE_SIZE. > Potentially larger, depending on whether the MM developers are willing to > guarantee that kmalloc(PAGE_SIZE * 2, GFP_KERNEL) will return a PAGE_SIZE > aligned block of memory indefinitely. Page size alignment of multi-page heap allocations is ncessary. The current behaviour w/ KASAN is to offset so a 8KB allocation spans 3 pages and is not page aligned. That causes just as much in way of alignment problems as unaligned objects in multi-object-per-page slabs. As I said in the lastest discussion of this problem on XFS (pmem devices w/ KASAN enabled), all we -need- is a GFP flag that tells the slab allocator to give us naturally aligned object or fail if it can't. I don't care how that gets implemented (e.g. another set of heap slabs like the -rcl slabs), I just don't want every high level subsystem that allocates heap memory for IO buffers to have to implement their own aligned slab caches. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
