linux-kernel - Re: [RFC 0/2] guarantee natural alignment for kmalloc()

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Date:   Wed, 20 Mar 2019 19:23:55 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Christopher Lameter <cl@...ux.com>, linux-mm@...ck.org,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Ming Lei <ming.lei@...hat.com>,
        Dave Chinner <david@...morbit.com>,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        Christoph Hellwig <hch@....de>,
        Michal Hocko <mhocko@...nel.org>, linux-kernel@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-block@...r.kernel.org
Subject: Re: [RFC 0/2] guarantee natural alignment for kmalloc()

On Wed, Mar 20, 2019 at 10:48:03PM +0100, Vlastimil Babka wrote:
> On 3/20/2019 7:53 PM, Matthew Wilcox wrote:
> > On Wed, Mar 20, 2019 at 09:48:47AM +0100, Vlastimil Babka wrote:
> >> Natural alignment to size is rather well defined, no? Would anyone ever
> >> assume a larger one, for what reason?
> >> It's now where some make assumptions (even unknowingly) for natural
> >> There are two 'odd' sizes 96 and 192, which will keep cacheline size
> >> alignment, would anyone really expect more than 64 bytes?
> > 
> > Presumably 96 will keep being aligned to 32 bytes, as aligning 96 to 64
> > just results in 128-byte allocations.
> 
> Well, looks like that's what happens. This is with SLAB, but the alignment
> calculations should be common: 
> 
> slabinfo - version: 2.1
> # name            <active_objs> <num_objs> <objsize> <objperslab> <pagesperslab> : tunables <limit> <batchcount> <sharedfactor> : slabdata <active_slabs> <num_slabs> <sharedavail>
> kmalloc-96          2611   4896    128   32    1 : tunables  120   60    8 : slabdata    153    153      0
> kmalloc-128         4798   5536    128   32    1 : tunables  120   60    8 : slabdata    173    173      0

Hmm.  On my laptop, I see:

kmalloc-96         28050  35364     96   42    1 : tunables    0    0    0 : slabdata    842    842      0

That'd take me from 842 * 4k pages to 1105 4k pages -- an extra megabyte of
memory.

This is running Debian's 4.19 kernel:

# CONFIG_SLAB is not set
CONFIG_SLUB=y
# CONFIG_SLOB is not set
CONFIG_SLAB_MERGE_DEFAULT=y
CONFIG_SLAB_FREELIST_RANDOM=y
CONFIG_SLAB_FREELIST_HARDENED=y
CONFIG_SLUB_CPU_PARTIAL=y