Re: [PATCH] mm: kfence: fix PG_slab and memcg_data clearing

[Muchun Song]

On 2023/3/21 12:14, Peng Zhang wrote:
> 在 2023/3/20 11:00, Muchun Song 写道:
> > It does not reset PG_slab and memcg_data when KFENCE fails to initialize
> > kfence pool at runtime. It is reporting a "Bad page state" message when
> > kfence pool is freed to buddy. The checking of whether it is a compound
> > head page seems unnecessary sicne we already guarantee this when 
> > allocating
> > kfence pool, removing the check to simplify the code.
> >
> > Fixes: 0ce20dd84089 ("mm: add Kernel Electric-Fence infrastructure")
> > Fixes: 8f0b36497303 ("mm: kfence: fix objcgs vector allocation")
> > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> > ---
> >   mm/kfence/core.c | 30 +++++++++++++++---------------
> >   1 file changed, 15 insertions(+), 15 deletions(-)
> >
> > diff --git a/mm/kfence/core.c b/mm/kfence/core.c
> > index 79c94ee55f97..d66092dd187c 100644
> > --- a/mm/kfence/core.c
> > +++ b/mm/kfence/core.c
> > @@ -561,10 +561,6 @@ static unsigned long kfence_init_pool(void)
> >           if (!i || (i % 2))
> >               continue;
> >   -        /* Verify we do not have a compound head page. */
> > -        if (WARN_ON(compound_head(&pages[i]) != &pages[i]))
> > -            return addr;
> > -
> >           __folio_set_slab(slab_folio(slab));
> >   #ifdef CONFIG_MEMCG
> >           slab->memcg_data = (unsigned long)&kfence_metadata[i / 2 - 
> > 1].objcg |
> > @@ -597,12 +593,26 @@ static unsigned long kfence_init_pool(void)
> >             /* Protect the right redzone. */
> >           if (unlikely(!kfence_protect(addr + PAGE_SIZE)))
> > -            return addr;
> > +            goto reset_slab;
> >             addr += 2 * PAGE_SIZE;
> >       }
> >         return 0;
> > +
> > +reset_slab:
> > +    for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) {
> > +        struct slab *slab = page_slab(&pages[i]);
> > +
> > +        if (!i || (i % 2))
> > +            continue;
> > +#ifdef CONFIG_MEMCG
> > +        slab->memcg_data = 0;
> > +#endif
> > +        __folio_clear_slab(slab_folio(slab));
> > +    }
> Can this loop be simplified to this?
>
>     for (i = 2; i < KFENCE_POOL_SIZE / PAGE_SIZE; i+=2) {
>         struct slab *slab = page_slab(&pages[i]);
> #ifdef CONFIG_MEMCG
>         slab->memcg_data = 0;
> #endif
>         __folio_clear_slab(slab_folio(slab));
>     }

It's a good simplification. The loop setting Pg_slab before this
also can be simplified in the same way. However, I choose a
consistent way to fix this bug. I'd like to send a separate
simplification patch to simplify both two loops instead of
in a bugfix patch.

Thanks.