Re: [PATCH v3 2/4] mm/hwpoison: check if a subpage of a hugetlb folio is raw HWPOISON

[Jiaqi Yan <jiaqiyan@google.com>]

On Fri, Jul 7, 2023 at 7:57 PM Miaohe Lin <linmiaohe@huawei.com> wrote:
>
> On 2023/7/8 4:19, Jiaqi Yan wrote:
> > Add the functionality, is_raw_hwp_subpage, to tell if a subpage of a
> > hugetlb folio is a raw HWPOISON page. This functionality relies on
> > RawHwpUnreliable to be not set; otherwise hugepage's raw HWPOISON list
> > becomes meaningless.
> >
> > is_raw_hwp_subpage needs to hold hugetlb_lock in order to synchronize
> > with __get_huge_page_for_hwpoison, who iterates and inserts an entry to
> > raw_hwp_list. llist itself doesn't ensure insertion is synchornized with
> > the iterating used by __is_raw_hwp_list. Caller can minimize the
> > overhead of lock cycles by first checking if folio / head page's
> > HWPOISON flag is set.
> >
> > Exports this functionality to be immediately used in the read operation
> > for hugetlbfs.
> >
> > Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
> > Reviewed-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
> > Signed-off-by: Jiaqi Yan <jiaqiyan@google.com>
> > ---
> >  include/linux/hugetlb.h | 19 +++++++++++++++++++
> >  include/linux/mm.h      |  7 +++++++
> >  mm/hugetlb.c            | 10 ++++++++++
> >  mm/memory-failure.c     | 34 ++++++++++++++++++++++++----------
> >  4 files changed, 60 insertions(+), 10 deletions(-)
> > ...
> > -static inline struct llist_head *raw_hwp_list_head(struct folio *folio)
> > +bool __is_raw_hwp_subpage(struct folio *folio, struct page *subpage)
> >  {
> > -     return (struct llist_head *)&folio->_hugetlb_hwpoison;
> > +     struct llist_head *raw_hwp_head;
> > +     struct raw_hwp_page *p, *tmp;
> > +     bool ret = false;
> > +
> > +     if (!folio_test_hwpoison(folio))
> > +             return false;
> > +
> > +     /*
> > +      * When RawHwpUnreliable is set, kernel lost track of which subpages
> > +      * are HWPOISON. So return as if ALL subpages are HWPOISONed.
> > +      */
> > +     if (folio_test_hugetlb_raw_hwp_unreliable(folio))
> > +             return true;
> > +
> > +     raw_hwp_head = raw_hwp_list_head(folio);
> > +     llist_for_each_entry_safe(p, tmp, raw_hwp_head->first, node) {
>
> Since we don't free the raw_hwp_list, does llist_for_each_entry works same as llist_for_each_entry_safe?
>
> > +             if (subpage == p->page) {
> > +                     ret = true;
> > +                     break;
> > +             }
> > +     }
> > +
> > +     return ret;
> >  }
>
> It seems there's a race between __is_raw_hwp_subpage and unpoison_memory:
>   unpoison_memory               __is_raw_hwp_subpage
>                                   if (!folio_test_hwpoison(folio)) -- hwpoison is set
>     folio_free_raw_hwp            llist_for_each_entry_safe raw_hwp_list
>       llist_del_all                 ..
>     folio_test_clear_hwpoison
>

Thanks Miaohe for raising this concern.

> But __is_raw_hwp_subpage is used in hugetlbfs, unpoison_memory couldn't reach here because there's a
> folio_mapping == NULL check before folio_free_raw_hwp.

I agree. But in near future I do want to make __is_raw_hwp_subpage
work for shared-mapping hugetlb, so it would be nice to work with
unpoison_memory. It doesn't seem to me that holding mf_mutex in
__is_raw_hwp_subpage is nice or even absolutely correct. Let me think
if I can come up with something in v4.

>
> Anyway, this patch looks good to me.
>
> Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>
> Thanks.
>