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From:   Jiaqi Yan <jiaqiyan@google.com>
Date:   Fri, 30 Jun 2023 13:59:23 -0700
Message-ID: <CACw3F52R8oUNP50dfy35m1KED82NKgKcHKk3ev4O+4nqhFVdzg@mail.gmail.com>
Subject: Re: [PATCH v2 1/4] mm/hwpoison: delete all entries before traversal
 in __folio_free_raw_hwp
To:     Naoya Horiguchi <naoya.horiguchi@linux.dev>
Cc:     mike.kravetz@oracle.com, naoya.horiguchi@nec.com,
        songmuchun@bytedance.com, shy828301@gmail.com,
        linmiaohe@huawei.com, akpm@linux-foundation.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        duenwen@google.com, axelrasmussen@google.com, jthoughton@google.com
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On Fri, Jun 30, 2023 at 7:52=E2=80=AFAM Naoya Horiguchi
<naoya.horiguchi@linux.dev> wrote:
>
> On Fri, Jun 23, 2023 at 04:40:12PM +0000, Jiaqi Yan wrote:
> > Traversal on llist (e.g. llist_for_each_safe) is only safe AFTER entrie=
s
> > are deleted from the llist.
> >
> > llist_del_all are lock free with itself. folio_clear_hugetlb_hwpoison()=
s
> > from __update_and_free_hugetlb_folio and memory_failure won't need
> > explicit locking when freeing the raw_hwp_list.
> >
> > Signed-off-by: Jiaqi Yan <jiaqiyan@google.com>
>
> (Sorry if stupid question...) folio_set_hugetlb_hwpoison() also calls
> llist_for_each_safe() but it still traverses the list without calling
> llist_del_all().  This convention applies only when removing item(s)?

I think in our previous discussion, Mike and I agree as of today's
code in hugetlb.c and memory-failure.c, concurrent adding, deleting,
traversing are fine with each other and with themselves [1], but new
code need to be careful wrt ops on raw_hwp_list.

This patch is a low-hanging fruit to ensure any caller of
__folio_free_raw_hwp won't introduce any problem by correcting one
thing in __folio_free_raw_hwp: since it wants to delete raw_hwp_page
entries in the list, it should do it by first llist_del_all, and then
kfree with a llist_for_each_safe.

As for folio_set_hugetlb_hwpoison, I am not very comfortable fixing
it. I imagine a way to fix it is llist_del_all() =3D>
llist_for_each_safe{...} =3D> llist_add_batch(), or llist_add() within
llist_for_each_safe{...}. I haven't really thought through if this is
a correct fix.

[1] https://lore.kernel.org/lkml/CACw3F51o1ZFSYZa+XLnk4Wwjy2w_q=3DKn+aOQs0=
=3DqpfG-ZYDFKg@mail.gmail.com/#t


>
> Thanks,
> Naoya Horiguchi
>
> > ---
> >  mm/memory-failure.c | 8 +++-----
> >  1 file changed, 3 insertions(+), 5 deletions(-)
> >
> > diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> > index 004a02f44271..c415c3c462a3 100644
> > --- a/mm/memory-failure.c
> > +++ b/mm/memory-failure.c
> > @@ -1825,12 +1825,11 @@ static inline struct llist_head *raw_hwp_list_h=
ead(struct folio *folio)
> >
> >  static unsigned long __folio_free_raw_hwp(struct folio *folio, bool mo=
ve_flag)
> >  {
> > -     struct llist_head *head;
> > -     struct llist_node *t, *tnode;
> > +     struct llist_node *t, *tnode, *head;
> >       unsigned long count =3D 0;
> >
> > -     head =3D raw_hwp_list_head(folio);
> > -     llist_for_each_safe(tnode, t, head->first) {
> > +     head =3D llist_del_all(raw_hwp_list_head(folio));
> > +     llist_for_each_safe(tnode, t, head) {
> >               struct raw_hwp_page *p =3D container_of(tnode, struct raw=
_hwp_page, node);
> >
> >               if (move_flag)
> > @@ -1840,7 +1839,6 @@ static unsigned long __folio_free_raw_hwp(struct =
folio *folio, bool move_flag)
> >               kfree(p);
> >               count++;
> >       }
> > -     llist_del_all(head);
> >       return count;
> >  }
> >
> > --
> > 2.41.0.162.gfafddb0af9-goog
> >
> >
> >