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

[Vlastimil Babka]

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@xxxxxx/
> [2] https://lore.kernel.org/linux-fsdevel/20190225040904.5557-1-ming.lei@xxxxxxxxxx/
> [3] https://lwn.net/Articles/787740/
> 
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>

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.

[1] https://lore.kernel.org/linux-mm/20190905214553.1643060-1-guro@xxxxxx/