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

[Kefeng Wang]

On 2021/9/27 15:22, Vlastimil Babka wrote:
> 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?

I want the check back in kfree, this one is used  widely in driver, and 
the probability

of problem occurred is bigger in driver, especially in some out of tree 
drivers.

> > 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);
> .