linux-kernel - Re: [PATCH v3 3/7] mm, slab: allocate off-slab freelists as reclaimable when appropriate

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Date:   Fri, 20 Jul 2018 11:37:34 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        Roman Gushchin <guro@...com>, Michal Hocko <mhocko@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v3 3/7] mm, slab: allocate off-slab freelists as
 reclaimable when appropriate

On 07/19/2018 10:35 AM, Mel Gorman wrote:
> On Wed, Jul 18, 2018 at 03:36:16PM +0200, Vlastimil Babka wrote:
>> In SLAB, OFF_SLAB caches allocate management structures (currently just the
>> freelist) from kmalloc caches when placement in a slab page together with
>> objects would lead to suboptimal memory usage. For SLAB_RECLAIM_ACCOUNT caches,
>> we can allocate the freelists from the newly introduced reclaimable kmalloc
>> caches, because shrinking the OFF_SLAB cache will in general result to freeing
>> of the freelists as well. This should improve accounting and anti-fragmentation
>> a bit.
>>
>> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> 
> I'm not quite convinced by this one. The freelist cache is tied to the
> lifetime of the slab and not the objects. A single freelist can be reclaimed
> eventually but for caches with many objects per slab, it could take a lot
> of shrinking random objects to reclaim one freelist. Functionally the
> patch appears to be fine.

Hm you're right that the reclaimability of freelist is maybe too much
detached, and could do more harm than good for the reclaimable caches. I
will probably drop it unless I can measure it's an improvement. Thanks.