Re: [PATCH v2] slub: Add back check for free nonslab objects

[Shakeel Butt <shakeelb@google.com>]

On Mon, Sep 27, 2021 at 5:24 AM Kefeng Wang <wangkefeng.wang@huawei.com> 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, add
> dump_page() and object address printing to help use to debug the issue.
>
> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
> ---
> v2: Add object address printing suggested by Matthew Wilcox
>
>  mm/slub.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 3095b889fab4..157973e22faf 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3522,7 +3522,11 @@ 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))) {

If there is a problem then this would be too noisy. Why not WARN_ON_ONCE()?

> +               dump_page(page, "invalid free nonslab page");
> +               pr_warn("object pointer: 0x%p\n", object);

Actually why not add 'once' semantics for the whole if-block?

> +       }
> +
>         kfree_hook(object);
>         mod_lruvec_page_state(page, NR_SLAB_UNRECLAIMABLE_B, -(PAGE_SIZE << order));
>         __free_pages(page, order);
> --
> 2.26.2
>