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

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Date:   Thu, 29 Aug 2019 08:24:22 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Christopher Lameter <cl@...ux.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Ming Lei <ming.lei@...hat.com>,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        Christoph Hellwig <hch@....de>, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        linux-btrfs@...r.kernel.org
Subject: Re: [PATCH v2 2/2] mm, sl[aou]b: guarantee natural alignment for
 kmalloc(power-of-two)

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@...morbit.com