Re: [PATCH v2 2/2] mm, sl[aou]b: guarantee natural alignment for kmalloc(power-of-two)

[Dave Chinner]

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