linux-kernel - Re: [PATCH 07/12] mm, slab: ignore SLAB_RECLAIM_ACCOUNT with CONFIG_SLUB

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Date:   Mon, 28 Nov 2022 00:11:12 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Pekka Enberg <penberg@...nel.org>
Cc:     Hyeonggon Yoo <42.hyeyoo@...il.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>, patches@...ts.linux.dev,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 07/12] mm, slab: ignore SLAB_RECLAIM_ACCOUNT with
 CONFIG_SLUB_TINY

On 11/21/22 18:11, Vlastimil Babka wrote:
> SLAB_RECLAIM_ACCOUNT caches allocate their slab pages with
> __GFP_RECLAIMABLE and can help against fragmentation by grouping pages
> by mobility, but on tiny systems mobility grouping is likely disabled
> anyway and ignoring SLAB_RECLAIM_ACCOUNT might instead lead to merging
> of caches that are made incompatible just by the flag.
> 
> Thus with CONFIG_SLUB_TINY, make SLAB_RECLAIM_ACCOUNT ineffective.
> 
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> ---
>  include/linux/slab.h | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 3ce9474c90ab..1cbbda03ad06 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -129,7 +129,11 @@
>  
>  /* The following flags affect the page allocator grouping pages by mobility */
>  /* Objects are reclaimable */
> +#ifndef CONFIG_SLUB_TINY
>  #define SLAB_RECLAIM_ACCOUNT	((slab_flags_t __force)0x00020000U)
> +#else
> +#define SLAB_RECLAIM_ACCOUNT	0

Updating the last line above to:
#define SLAB_RECLAIM_ACCOUNT      ((slab_flags_t __force)0)

In response to:
https://lore.kernel.org/all/202211280441.yCEecX9z-lkp@intel.com/

Yeah it probably means that the other pre-existing flag variants that
#define to 0 should be also adjusted to avoid these issues, but not as part
of this series.

> +#endif
>  #define SLAB_TEMPORARY		SLAB_RECLAIM_ACCOUNT	/* Objects are short-lived */
>  
>  /*