Re: [PATCH resend] slub: Add back check for free nonslab objects

[Vlastimil Babka]

On 9/27/21 04:15, Kefeng Wang wrote:
> After commit ("f227f0faf63b slub: fix unreclaimable slab stat for bulk
> free"), the check for free nonslab page is replaced by VM_BUG_ON_PAGE,
> which only check with CONFIG_DEBUG_VM enabled, but this config may
> impact performance, so it only for debug.
> 
> Commit ("0937502af7c9 slub: Add check for kfree() of non slab objects.")
> add the ability, which should be needed in any configs to catch the
> invalid free, they even could be potential issue, eg, memory corruption,
> use after free and double-free, so replace VM_BUG_ON_PAGE to WARN_ON, and
> add dump_page() to help use to debug the issue.

There are other situations in SLUB (such as with smaller allocations that
don't go directly to page allocator) where use after free and double-free
are undetected in non-debug configs, and it's expected that anyone debugging
them will enable slub_debug or even DEBUG_VM. Why should this special case
with nonslab pages be different?

> Signed-off-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
> ---
>  mm/slub.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 3095b889fab4..555c2e6ccfca 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3522,7 +3522,8 @@ static inline void free_nonslab_page(struct page *page, void *object)
>  {
>  	unsigned int order = compound_order(page);
>  
> -	VM_BUG_ON_PAGE(!PageCompound(page), page);
> +	if (WARN_ON(!PageCompound(page)))
> +		dump_page(page, "invalid free nonslab page");
>  	kfree_hook(object);
>  	mod_lruvec_page_state(page, NR_SLAB_UNRECLAIMABLE_B, -(PAGE_SIZE << order));
>  	__free_pages(page, order);
>