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From: Vlastimil Babka <>
To: Christopher Lameter <>
Cc:, Pekka Enberg <>,
	David Rientjes <>,
	Joonsoo Kim <>,
	Ming Lei <>,
	Dave Chinner <>,
	Matthew Wilcox <>,
	"Darrick J . Wong" <>,
	Christoph Hellwig <>, Michal Hocko <>,,,,
Subject: Re: [RFC 0/2] guarantee natural alignment for kmalloc()
Date: Thu, 21 Mar 2019 08:42:02 +0100	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On 3/20/19 7:20 PM, Christopher Lameter wrote:
>>> Currently all kmalloc objects are aligned to KMALLOC_MIN_ALIGN. That will
>>> no longer be the case and alignments will become inconsistent.
>> KMALLOC_MIN_ALIGN is still the minimum, but in practice it's larger
>> which is not a problem.
> "In practice" refers to the current way that slab allocators arrange
> objects within the page. They are free to do otherwise if new ideas come
> up for object arrangements etc.
> The slab allocators already may have to store data in addition to the user
> accessible part (f.e. for RCU or ctor). The "natural alighnment" of a
> power of 2 cache is no longer as you expect for these cases. Debugging is
> not the only case where we extend the object.

For plain kmalloc() caches, RCU and ctors don't apply, right.

>> Also let me stress again that nothing really changes except for SLOB,
>> and SLUB with debug options. The natural alignment for power-of-two
>> sizes already happens as SLAB and SLUB both allocate objects starting on
>> the page boundary. So people make assumptions based on that, and then
>> break with SLOB, or SLUB with debug. This patch just prevents that
>> breakage by guaranteeing those natural assumptions at all times.
> As explained before there is nothing "natural" here. Doing so restricts
> future features

Well, future features will have to deal with the existing named caches
created with specific alignment.

> and creates a mess within the allocator of exceptions for
> debuggin etc etc (see what happened to SLAB).

SLAB could be fixed, just nobody cares enough I guess. If I want to
debug wrong SL*B usage I'll use SLUB.

> "Natural" is just a
> simplistic thought of a user how he would arrange power of 2 objects.
> These assumption should not be made but specified explicitly.

Patch 1 does this explicitly for plain kmalloc(). It's unrealistic to
add 'align' parameter to plain kmalloc() as that would have to create
caches on-demand for 'new' values of align parameter.

>>> I think its valuable that alignment requirements need to be explicitly
>>> requested.
>> That's still possible for named caches created by kmem_cache_create().
> So lets leave it as it is now then.

That however doesn't work well for the xfs/IO case where block sizes are
not known in advance:

  reply	other threads:[~2019-03-21  7:42 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-03-19 21:11 [RFC 0/2] guarantee natural alignment for kmalloc() Vlastimil Babka
2019-03-19 21:11 ` [RFC 1/2] mm, sl[aou]b: guarantee natural alignment for kmalloc(power-of-two) Vlastimil Babka
2019-03-19 21:11 ` [RFC 2/2] mm, sl[aou]b: test whether kmalloc() alignment works as expected Vlastimil Babka
2019-03-20  0:44   ` Christopher Lameter
2019-03-20  0:43 ` [RFC 0/2] guarantee natural alignment for kmalloc() Christopher Lameter
2019-03-20  0:53   ` David Rientjes
2019-03-20  8:48   ` Vlastimil Babka
2019-03-20 18:20     ` Christopher Lameter
2019-03-21  7:42       ` Vlastimil Babka [this message]
2019-03-22 17:52         ` Christopher Lameter
2019-04-05 17:11           ` Vlastimil Babka
2019-04-07  8:00             ` Christoph Hellwig
2019-04-09  8:07               ` Vlastimil Babka
2019-04-09  9:20                 ` Michal Hocko
2019-03-20 18:53     ` Matthew Wilcox
2019-03-20 21:48       ` Vlastimil Babka
2019-03-21  2:23         ` Matthew Wilcox
2019-03-21  7:02           ` Vlastimil Babka

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