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

[Kefeng Wang]

On 2021/9/28 23:09, Shakeel Butt wrote:
> On Mon, Sep 27, 2021 at 5:24 AM Kefeng Wang <wangkefeng.wang@xxxxxxxxxx> 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@xxxxxxxxxx>
> > ---
> > 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()?

If lots of abnormal/illegal pages are freed to freelist, the system
could be crash much more easier, with that in mind, I think the original 
logical use BUG_ON().

The ksize() use WARN_ON, looks no one report about too much log.

If we don't want too much dump, I will change it in v3.

> > +               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
> .