Re: [RFC PATCH] mm/slab: save memory by allocating slabobj_ext array from leftover

[Yeoreum Yun <yeoreum.yun@arm.com>]

Hi Harry,

[...]
> Allocate slabobj_exts array from this unused space instead of using
> kcalloc(), when it is large enough.
>
> Enjoy the memory savings!
>
> [ MEMCG=y, MEM_ALLOC_PROFILING=y ]
>
> Before patch (run updatedb):
>   Slab:            5815196 kB
>   SReclaimable:    5042824 kB
>   SUnreclaim:       772372 kB
>
> After patch (run updatedb):
>   Slab:            5748664 kB
>   SReclaimable:    5041608 kB
>   SUnreclaim:       707084 kB (-63.75 MiB)
>
> [ MEMCG=y, MEM_ALLOC_PROFILING=n ]
>
> Before patch (run updatedb):
>   Slab:            5637764 kB
>   SReclaimable:    5042428 kB
>   SUnreclaim:       595284 kB
>
> After patch (run updatedb):
>   Slab:            5598992 kB
>   SReclaimable:    5042248 kB
>   SUnreclaim:       560396 kB (-34.07 MiB)
>
> This saves from hundreds of KiBs up to several tens of MiBs of memory
> on my machine, depending on the config and slab memory usage.
>
> Enjoy the memory savings!

Awesome :)

[...]
>  #ifdef CONFIG_SLUB_DEBUG
>  static unsigned long object_map[BITS_TO_LONGS(MAX_OBJS_PER_PAGE)];
>  static DEFINE_SPINLOCK(object_map_lock);
> @@ -1307,7 +1350,15 @@ slab_pad_check(struct kmem_cache *s, struct slab *slab)
>       start = slab_address(slab);
>       length = slab_size(slab);
>       end = start + length;
> -     remainder = length % s->size;
> +
> +     if (can_alloc_obj_exts_from_leftover(s, slab)) {
> +             remainder = length;
> +             remainder -= obj_exts_offset(s, slab);
> +             remainder -= obj_exts_size(slab);
> +     } else {
> +             remainder = length % s->size;
> +     }
> +
>       if (!remainder)
>               return;
>
> @@ -2049,6 +2100,21 @@ static noinline void free_slab_obj_exts(struct slab *slab)
>       slab->obj_exts = 0;
>  }

What concerns me about this patch is the case where !memcg_kmem_online() and
MEM_ALLOC_PROFILING is not used.
With this patch, obj_ext can still be created even in that situation,
and as a result, if data is overwritten in the region previously padded with
POISON_INUSE (before the patch), slab_pad_check() may no longer catch it

If this's ignorable, feel free toadd :

Reviewed-by: Yeoreum Yun <yeoreum.yun@arm.com>

--
Sincerely,
Yeoreum Yun
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