Re: [patch 6/7] slub: fix unreclaimable slab stat for bulk free

[Nathan Chancellor <nathan@kernel.org>]

On Thu, Jul 29, 2021 at 02:53:50PM -0700, Andrew Morton wrote:
> From: Shakeel Butt <shakeelb@google.com>
> Subject: slub: fix unreclaimable slab stat for bulk free
> 
> SLUB uses page allocator for higher order allocations and update
> unreclaimable slab stat for such allocations.  At the moment, the bulk
> free for SLUB does not share code with normal free code path for these
> type of allocations and have missed the stat update.  So, fix the stat
> update by common code.  The user visible impact of the bug is the
> potential of inconsistent unreclaimable slab stat visible through meminfo
> and vmstat.
> 
> Link: https://lkml.kernel.org/r/20210728155354.3440560-1-shakeelb@google.com
> Fixes: 6a486c0ad4dc ("mm, sl[ou]b: improve memory accounting")
> Signed-off-by: Shakeel Butt <shakeelb@google.com>
> Acked-by: Michal Hocko <mhocko@suse.com>
> Acked-by: Roman Gushchin <guro@fb.com>
> Reviewed-by: Muchun Song <songmuchun@bytedance.com>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Pekka Enberg <penberg@kernel.org>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
> 
>  mm/slub.c |   22 ++++++++++++----------
>  1 file changed, 12 insertions(+), 10 deletions(-)
> 
> --- a/mm/slub.c~slub-fix-unreclaimable-slab-stat-for-bulk-free
> +++ a/mm/slub.c
> @@ -3236,6 +3236,16 @@ struct detached_freelist {
>  	struct kmem_cache *s;
>  };
>  
> +static inline void free_nonslab_page(struct page *page)
> +{
> +	unsigned int order = compound_order(page);
> +
> +	VM_BUG_ON_PAGE(!PageCompound(page), page);
> +	kfree_hook(page_address(page));
> +	mod_lruvec_page_state(page, NR_SLAB_UNRECLAIMABLE_B, -(PAGE_SIZE << order));
> +	__free_pages(page, order);
> +}
> +
>  /*
>   * This function progressively scans the array with free objects (with
>   * a limited look ahead) and extract objects belonging to the same
> @@ -3272,9 +3282,7 @@ int build_detached_freelist(struct kmem_
>  	if (!s) {
>  		/* Handle kalloc'ed objects */
>  		if (unlikely(!PageSlab(page))) {
> -			BUG_ON(!PageCompound(page));
> -			kfree_hook(object);
> -			__free_pages(page, compound_order(page));
> +			free_nonslab_page(page);
>  			p[size] = NULL; /* mark object processed */
>  			return size;
>  		}
> @@ -4250,13 +4258,7 @@ void kfree(const void *x)
>  
>  	page = virt_to_head_page(x);
>  	if (unlikely(!PageSlab(page))) {
> -		unsigned int order = compound_order(page);
> -
> -		BUG_ON(!PageCompound(page));
> -		kfree_hook(object);
> -		mod_lruvec_page_state(page, NR_SLAB_UNRECLAIMABLE_B,
> -				      -(PAGE_SIZE << order));
> -		__free_pages(page, order);
> +		free_nonslab_page(page);
>  		return;
>  	}
>  	slab_free(page->slab_cache, page, object, NULL, 1, _RET_IP_);
> _

This patch, now in mainline as commit f227f0faf63b ("slub: fix
unreclaimable slab stat for bulk free") causes the KASAN KUnit test
kmalloc_pagealloc_invalid_free to no longer fail:

[    0.000000] Linux version 5.14.0-rc3-00066-gf227f0faf63b (nathan@archlinux-ax161) (x86_64-linux-gcc (GCC) 11.2.0, GNU ld (GNU Binutils) 2.37) #1 SMP Sat Jul 31 15:08:11 MST 2021
...
[    5.717678]     # kmalloc_pagealloc_invalid_free: EXPECTATION FAILED at lib/test_kasan.c:203
[    5.717678]     KASAN failure expected in "kfree(ptr + 1)", but none occurred
[    5.718909]     not ok 6 - kmalloc_pagealloc_invalid_free
...
[    9.481520] not ok 1 - kasan

The previous commit is fine:

[    0.000000] Linux version 5.14.0-rc3-00065-gb5916c025432 (nathan@archlinux-ax161) (x86_64-linux-gcc (GCC) 11.2.0, GNU ld (GNU Binutils) 2.37) #1 SMP Sat Jul 31 15:05:09 MST 2021
...
[    9.347598] ok 1 - kasan

I am by no means a KASAN or mm/ expert, I noticed this when trying to
test KASAN with clang for ClangBuiltLinux's CI, so it does not appear to
be compiler dependent. It is reproducible for me in QEMU with
x86_64_defconfig + CONFIG_KASAN=y + CONFIG_KUNIT=y +
CONFIG_KASAN_KUNIT_TEST=y.

Please let me know if there is any other information I can provide or
testing I can do.

Cheers,
Nathan