From: Roman Gushchin <guro@fb.com>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: Andrew Morton <akpm@linux-foundation.org>,
"linux-mm@kvack.org" <linux-mm@kvack.org>,
"linux-kernel@vger.kernel.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-xfs@vger.kernel.org>,
"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
"linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
James Bottomley <James.Bottomley@hansenpartnership.com>,
"linux-btrfs@vger.kernel.org" <linux-btrfs@vger.kernel.org>,
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, 23 Sep 2019 17:54:01 +0000 [thread overview]
Message-ID: <20190923175356.GB3644@tower.DHCP.thefacebook.com> (raw)
In-Reply-To: <df8d1cf4-ff8f-1ee1-12fb-cfec39131b32@suse.cz>
On Mon, Sep 23, 2019 at 06:36:32PM +0200, 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://urldefense.proofpoint.com/v2/url?u=https-3A__lwn.net_Articles_787740_&d=DwICaQ&c=5VD0RTtNlTh3ycd41b3MUw&r=jJYgtDM7QT-W-Fz_d29HYQ&m=UUX1YoPTOOycNowHuRP2ZnqwSwZFjAFrkQFrqstidZ0&s=Kt_XTKlh2qxbC_7ME44MV3_QWFVRHlI1p2EQZYP0uqY&e=
> >
> > 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).
>
> Otherwise I don't think there can be an objective conclusion. On the one
> hand we avoid further problems and workarounds due to misalignment (or
> objects allocated beyond page boundary, which was only recently
> mentioned), on the other hand we potentially make future changes to
> SLAB/SLUB or hypotetical new implementation either more complicated, or
> less effective due to extra fragmentation. Different people can have
> different opinions on what's more important.
>
> Let me however explain why I think we don't have to fear the future
> implementation complications that much. There was an argument IIRC that
> extra non-debug metadata could start to be prepended/appended to an
> object in the future (i.e. RCU freeing head?).
>
> 1) Caches can be already created with explicit alignment, so a naive
> pre/appending implementation would already waste memory on such caches.
> 2) Even without explicit alignment, a single slab cache for 512k objects
> with few bytes added to each object would waste almost 512k as the
> objects wouldn't fit precisely in an (order-X) page. The percentage
> wasted depends on X.
> 3) Roman recently posted a patchset [1] that basically adds a cgroup
> pointer to each object. The implementation doesn't append it to objects
> naively however, but adds a separately allocated array. Alignment is
> thus unchanged.
To be fair here, we *might* want to put this pointer just after/before
the object to reduce the number of cache misses. I've put it into
a separate place mostly for simplicity reasons.
It's not an objection though, just a note.
Thanks!
>
> [1] https://lore.kernel.org/linux-mm/20190905214553.1643060-1-guro@fb.com/
>
next prev parent reply other threads:[~2019-09-23 17:54 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 [this message]
2019-09-30 8:06 ` Christoph Hellwig
2019-09-30 9:23 ` Michal Hocko
2019-09-30 9:32 ` Kirill A. Shutemov
2019-09-23 18:57 ` Matthew Wilcox
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