Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Thu, 29 Aug 2019 08:24:22 +1000
From:   Dave Chinner <david@fromorbit.com>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Christopher Lameter <cl@linux.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Ming Lei <ming.lei@redhat.com>,
        "Darrick J . Wong" <darrick.wong@oracle.com>,
        Christoph Hellwig <hch@lst.de>, linux-xfs@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
        James Bottomley <James.Bottomley@hansenpartnership.com>,
        linux-btrfs@vger.kernel.org
Subject: Re: [PATCH v2 2/2] mm, sl[aou]b: guarantee natural alignment for
 kmalloc(power-of-two)
Message-ID: <20190828222422.GL1119@dread.disaster.area>
References: <20190826111627.7505-1-vbabka@suse.cz>
 <20190826111627.7505-3-vbabka@suse.cz>
 <0100016cd98bb2c1-a2af7539-706f-47ba-a68e-5f6a91f2f495-000000@email.amazonses.com>
 <20190828194607.GB6590@bombadil.infradead.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20190828194607.GB6590@bombadil.infradead.org>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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@fromorbit.com