Re: [PATCH 06/12] mm, slub: don't create kmalloc-rcl caches with CONFIG_SLUB_TINY

[Hyeonggon Yoo]

On Thu, Nov 24, 2022 at 01:12:13PM +0100, Vlastimil Babka wrote:
> On 11/24/22 13:06, Hyeonggon Yoo wrote:
> > On Wed, Nov 23, 2022 at 02:53:43PM +0100, Vlastimil Babka wrote:
> >> On 11/21/22 18:11, Vlastimil Babka wrote:
> >> > Distinguishing kmalloc(__GFP_RECLAIMABLE) can help against fragmentation
> >> > by grouping pages by mobility, but on tiny systems the extra memory
> >> > overhead of separate set of kmalloc-rcl caches will probably be worse,
> >> > and mobility grouping likely disabled anyway.
> >> > 
> >> > Thus with CONFIG_SLUB_TINY, don't create kmalloc-rcl caches and use the
> >> > regular ones.
> >> > 
> >> > Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> >> 
> >> Fixed up in response to lkp report for a MEMCG_KMEM+SLUB_TINY combo:
> >> ---8<---
> >> From c1ec0b924850a2863d061f316615d596176f15bb Mon Sep 17 00:00:00 2001
> >> From: Vlastimil Babka <vbabka@xxxxxxx>
> >> Date: Tue, 15 Nov 2022 18:19:28 +0100
> >> Subject: [PATCH 06/12] mm, slub: don't create kmalloc-rcl caches with
> >>  CONFIG_SLUB_TINY
> >> 
> >> Distinguishing kmalloc(__GFP_RECLAIMABLE) can help against fragmentation
> >> by grouping pages by mobility, but on tiny systems the extra memory
> >> overhead of separate set of kmalloc-rcl caches will probably be worse,
> >> and mobility grouping likely disabled anyway.
> >> 
> >> Thus with CONFIG_SLUB_TINY, don't create kmalloc-rcl caches and use the
> >> regular ones.
> >> 
> >> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> >> ---
> >>  include/linux/slab.h |  9 +++++++--
> >>  mm/slab_common.c     | 10 ++++++++--
> >>  2 files changed, 15 insertions(+), 4 deletions(-)
> >> 
> >> diff --git a/include/linux/slab.h b/include/linux/slab.h
> >> index 45efc6c553b8..ae2d19ec8467 100644
> >> --- a/include/linux/slab.h
> >> +++ b/include/linux/slab.h
> >> @@ -336,12 +336,17 @@ enum kmalloc_cache_type {
> >>  #endif
> >>  #ifndef CONFIG_MEMCG_KMEM
> >>  	KMALLOC_CGROUP = KMALLOC_NORMAL,
> >> -#else
> >> -	KMALLOC_CGROUP,
> >>  #endif
> >> +#ifdef CONFIG_SLUB_TINY
> >> +	KMALLOC_RECLAIM = KMALLOC_NORMAL,
> >> +#else
> >>  	KMALLOC_RECLAIM,
> >> +#endif
> >>  #ifdef CONFIG_ZONE_DMA
> >>  	KMALLOC_DMA,
> >> +#endif
> >> +#ifdef CONFIG_MEMCG_KMEM
> >> +	KMALLOC_CGROUP,
> >>  #endif
> >>  	NR_KMALLOC_TYPES
> >>  };
> > 
> > Can you please elaborate what the lkp report was about
> > and how you fixed it? I'm not getting what the problem of previous
> > version is.
> 
> Report here:
> https://lore.kernel.org/all/202211231949.nIyAWKam-lkp@xxxxxxxxx/
> 
> Problem is that if the preprocessing results in e.g.
> KMALLOC_NORMAL = 0,
> KMALLOC_DMA = KMALLOC_NORMAL
> KMALLOC_CGROUP,
> KMALLOC_RECLAIM = KMALLOC_NORMAL,
> NR_KMALLOC_TYPES
> 
> then NR_KMALLOC_TYPES is not 2, but 1, because the enum's internal counter
> got reset to 0 by KMALLOC_RECLAIM = KMALLOC_NORMAL. A common gotcha :/

Thanks for quick and kind explanation :)
That was easy to be missed.

-- 
Thanks,
Hyeonggon