From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Michal Hocko <mhocko@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>,
Andrew Morton <akpm@linux-foundation.org>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Christoph Lameter <cl@linux.com>,
Pekka Enberg <penberg@kernel.org>,
David Rientjes <rientjes@google.com>,
Ming Lei <ming.lei@redhat.com>,
Dave Chinner <david@fromorbit.com>,
Matthew Wilcox <willy@infradead.org>,
"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, Roman Gushchin <guro@fb.com>,
Johannes Weiner <hannes@cmpxchg.org>
Subject: Re: [PATCH v2 2/2] mm, sl[aou]b: guarantee natural alignment for kmalloc(power-of-two)
Date: Mon, 30 Sep 2019 12:32:33 +0300 [thread overview]
Message-ID: <20190930093233.jlypzgmkf4pplgso@box.shutemov.name> (raw)
In-Reply-To: <20190930092334.GA25306@dhcp22.suse.cz>
On Mon, Sep 30, 2019 at 11:23:34AM +0200, Michal Hocko wrote:
> On Mon 23-09-19 18:36:32, Vlastimil Babka wrote:
> > On 8/26/19 1:16 PM, Vlastimil Babka wrote:
> > > In most configurations, kmalloc() happens to return naturally aligned (i.e.
> > > aligned to the block size itself) blocks for power of two sizes. That means
> > > some kmalloc() users might unknowingly rely on that alignment, until stuff
> > > breaks when the kernel is built with e.g. CONFIG_SLUB_DEBUG or CONFIG_SLOB,
> > > and blocks stop being aligned. Then developers have to devise workaround such
> > > as own kmem caches with specified alignment [1], which is not always practical,
> > > as recently evidenced in [2].
> > >
> > > The topic has been discussed at LSF/MM 2019 [3]. Adding a 'kmalloc_aligned()'
> > > variant would not help with code unknowingly relying on the implicit alignment.
> > > For slab implementations it would either require creating more kmalloc caches,
> > > or allocate a larger size and only give back part of it. That would be
> > > wasteful, especially with a generic alignment parameter (in contrast with a
> > > fixed alignment to size).
> > >
> > > Ideally we should provide to mm users what they need without difficult
> > > workarounds or own reimplementations, so let's make the kmalloc() alignment to
> > > size explicitly guaranteed for power-of-two sizes under all configurations.
> > > What this means for the three available allocators?
> > >
> > > * SLAB object layout happens to be mostly unchanged by the patch. The
> > > implicitly provided alignment could be compromised with CONFIG_DEBUG_SLAB due
> > > to redzoning, however SLAB disables redzoning for caches with alignment
> > > larger than unsigned long long. Practically on at least x86 this includes
> > > kmalloc caches as they use cache line alignment, which is larger than that.
> > > Still, this patch ensures alignment on all arches and cache sizes.
> > >
> > > * SLUB layout is also unchanged unless redzoning is enabled through
> > > CONFIG_SLUB_DEBUG and boot parameter for the particular kmalloc cache. With
> > > this patch, explicit alignment is guaranteed with redzoning as well. This
> > > will result in more memory being wasted, but that should be acceptable in a
> > > debugging scenario.
> > >
> > > * SLOB has no implicit alignment so this patch adds it explicitly for
> > > kmalloc(). The potential downside is increased fragmentation. While
> > > pathological allocation scenarios are certainly possible, in my testing,
> > > after booting a x86_64 kernel+userspace with virtme, around 16MB memory
> > > was consumed by slab pages both before and after the patch, with difference
> > > in the noise.
> > >
> > > [1] https://lore.kernel.org/linux-btrfs/c3157c8e8e0e7588312b40c853f65c02fe6c957a.1566399731.git.christophe.leroy@c-s.fr/
> > > [2] https://lore.kernel.org/linux-fsdevel/20190225040904.5557-1-ming.lei@redhat.com/
> > > [3] https://lwn.net/Articles/787740/
> > >
> > > Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> >
> > So if anyone thinks this is a good idea, please express it (preferably
> > in a formal way such as Acked-by), otherwise it seems the patch will be
> > dropped (due to a private NACK, apparently).
>
> Sigh.
>
> An existing code to workaround the lack of alignment guarantee just show
> that this is necessary. And there wasn't any real technical argument
> against except for a highly theoretical optimizations/new allocator that
> would be tight by the guarantee.
>
> Therefore
> Acked-by: Michal Hocko <mhocko@suse.com>
Agreed.
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
--
Kirill A. Shutemov
next prev parent reply other threads:[~2019-09-30 9:32 UTC|newest]
Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-26 11:16 [PATCH v2 0/2] guarantee natural alignment for kmalloc() Vlastimil Babka
2019-08-26 11:16 ` [PATCH v2 1/2] mm, sl[ou]b: improve memory accounting Vlastimil Babka
2019-08-26 11:16 ` [PATCH v2 2/2] mm, sl[aou]b: guarantee natural alignment for kmalloc(power-of-two) Vlastimil Babka
2019-08-28 18:45 ` Christopher Lameter
2019-08-28 19:46 ` Matthew Wilcox
2019-08-28 22:24 ` Dave Chinner
2019-08-29 7:56 ` Vlastimil Babka
2019-08-30 0:29 ` Dave Chinner
2019-08-29 7:39 ` Michal Hocko
2019-08-30 17:41 ` Christopher Lameter
2019-09-01 0:52 ` Matthew Wilcox
2019-09-03 20:13 ` Christopher Lameter
2019-09-03 20:53 ` Matthew Wilcox
2019-09-04 5:19 ` Christoph Hellwig
2019-09-04 6:40 ` Ming Lei
2019-09-04 7:20 ` Vlastimil Babka
2019-09-04 19:31 ` Christopher Lameter
2019-09-23 16:36 ` Vlastimil Babka
2019-09-23 17:17 ` David Sterba
2019-09-23 17:51 ` Darrick J. Wong
2019-09-24 20:47 ` cl
2019-09-24 20:51 ` Matthew Wilcox
2019-09-24 20:55 ` cl
2019-09-26 13:02 ` David Sterba
2019-09-24 21:19 ` Vlastimil Babka
2019-09-24 21:53 ` Dave Chinner
2019-09-24 22:21 ` Darrick J. Wong
2019-09-24 20:52 ` cl
2019-09-24 23:54 ` Andrew Morton
2019-09-25 7:17 ` Vlastimil Babka
2019-09-26 0:16 ` Christopher Lameter
2019-09-26 0:14 ` Christopher Lameter
2019-09-26 7:41 ` Vlastimil Babka
2019-09-28 1:12 ` Christopher Lameter
2019-09-30 13:32 ` Matthew Wilcox
2019-09-23 17:54 ` Roman Gushchin
2019-09-30 8:06 ` Christoph Hellwig
2019-09-30 9:23 ` Michal Hocko
2019-09-30 9:32 ` Kirill A. Shutemov [this message]
2019-09-23 18:57 ` Matthew Wilcox
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