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From: Vlastimil Babka <>
To: James Bottomley <>,
Cc: Linux-FSDevel <>,
	linux-mm <>,, Michal Hocko <>,
	Christoph Lameter <>,
	David Rientjes <>,
	Pekka Enberg <>,
	Joonsoo Kim <>,
	Ming Lei <>,, Christoph Hellwig <>,
	Dave Chinner <>,
	"Darrick J . Wong" <>
Subject: Re: [LSF/MM TOPIC] guarantee natural alignment for kmalloc()?
Date: Fri, 12 Apr 2019 09:54:07 +0200	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On 4/12/19 9:14 AM, James Bottomley wrote:
>> In the session I hope to resolve the question whether this is indeed
>> the right thing to do for all kmalloc() users, without an explicit
>> alignment requests, and if it's worth the potentially worse
>> performance/fragmentation it would impose on a hypothetical new slab
>> implementation for which it wouldn't be optimal to split power-of-two
>> sized pages into power-of-two-sized objects (or whether there are any
>> other downsides).
> I think so.  The question is how aligned?  explicit flushing arch's
> definitely need at least cache line alignment when using kmalloc for
> I/O and if allocations cross cache lines they have serious coherency
> problems.   The question of how much more aligned than this is
> interesting ... I've got to say that the power of two allocator implies
> same alignment as size and we seem to keep growing use cases that
> assume this.

Right, by "natural alignment" I meant exactly that - align to size for
power-of-two sizes.

> I'm not so keen on growing a separate API unless there's
> a really useful mm efficiency in breaking the kmalloc alignment
> assumptions.

I'd argue there's not.

> James

  reply	other threads:[~2019-04-12  7:57 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-04-11 12:52 [LSF/MM TOPIC] guarantee natural alignment for kmalloc()? Vlastimil Babka
2019-04-11 13:28 ` Matthew Wilcox
2019-04-25 11:33   ` Matthew Wilcox
2019-04-25 12:03     ` Martin K. Petersen
2019-04-25 12:03     ` Michal Hocko
2019-04-12  7:14 ` James Bottomley
2019-04-12  7:54   ` Vlastimil Babka [this message]
2019-04-16 15:38     ` Christopher Lameter
2019-04-17  8:07       ` Vlastimil Babka

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