linux-kernel - Re: [PATCH v4] slob: add size header to all allocations

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Date:   Mon, 22 Nov 2021 12:49:35 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Christoph Lameter <cl@...two.org>
Cc:     Rustam Kovhaev <rkovhaev@...il.com>, penberg@...nel.org,
        rientjes@...gle.com, iamjoonsoo.kim@....com,
        akpm@...ux-foundation.org, corbet@....net, djwong@...nel.org,
        david@...morbit.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-doc@...r.kernel.org,
        gregkh@...uxfoundation.org, viro@...iv.linux.org.uk,
        dvyukov@...gle.com
Subject: Re: [PATCH v4] slob: add size header to all allocations

On 11/22/21 12:40, Christoph Lameter wrote:
> On Mon, 22 Nov 2021, Vlastimil Babka wrote:
> 
>> It's no problem to have the real beginning of the object aligned, and the
>> prepended header not. The code already does that before this patch for the
>> kmalloc power-of-two alignments, where e.g. the object can be aligned to 256
>> bytes, but the prepended header to a smaller ARCH_KMALLOC_MINALIGN /
>> ARCH_SLAB_MINALIGN.
> 
> Ok but then the first object in a page may still have those issues.

Hmm, that's right. I guess we should also distinguish ARCH_KMALLOC_MINALIGN
for kmalloc paths, and ARCH_SLAB_MINALIGN for kmem_cache_alloc paths. Seems
the latter is generally smaller, thus some holes left by kmalloc allocations
could be filled later by kmem_cache_alloc allocations.

> 
>> > I dont know but it seems that making slob that sophisticated is counter
>> > productive. Remove SLOB?
>>
>> I wouldn't mind, but somebody might :)
> 
> Well run a space efficiency analysis after this patch. If the memory used
> is larger than SLUB (with the configuration for minimal data footprint)
> then there is no reason for SLOB to continue.

Makes sense.