On Fri, Jun 30, 2023 at 7:52 AM Naoya Horiguchi
<naoya.horiguchi@xxxxxxxxx> 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 entries
> > 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@xxxxxxxxxx>
>
> (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() =>
llist_for_each_safe{...} => 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=Kn+aOQs0=qpfG-ZYDFKg@xxxxxxxxxxxxxx/#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_head(struct folio *folio)
> >
> > static unsigned long __folio_free_raw_hwp(struct folio *folio, bool move_flag)
> > {
> > - struct llist_head *head;
> > - struct llist_node *t, *tnode;
> > + struct llist_node *t, *tnode, *head;
> > unsigned long count = 0;
> >
> > - head = raw_hwp_list_head(folio);
> > - llist_for_each_safe(tnode, t, head->first) {
> > + head = llist_del_all(raw_hwp_list_head(folio));
> > + llist_for_each_safe(tnode, t, head) {
> > struct raw_hwp_page *p = 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
> >
> >
> >
