Re: [PATCH 4/4] mm/memory-failure.c: fix potential VM_BUG_ON_PAGE in split_huge_page_to_list

[Yang Shi <shy828301@gmail.com>]

On Tue, Mar 8, 2022 at 4:36 AM Miaohe Lin <linmiaohe@huawei.com> wrote:
>
> On 2022/3/8 3:53, Yang Shi wrote:
> > On Sun, Mar 6, 2022 at 11:07 PM Miaohe Lin <linmiaohe@huawei.com> wrote:
> >>
> >> On 2022/3/4 16:28, HORIGUCHI NAOYA(堀口 直也) wrote:
> >>> On Mon, Feb 28, 2022 at 10:02:45PM +0800, Miaohe Lin wrote:
> >>>> The huge zero page could reach here and if we ever try to split it, the
> >>>> VM_BUG_ON_PAGE will be triggered in split_huge_page_to_list(). Also the
> >>>> non-lru compound movable pages could be taken for transhuge pages. Skip
> >>>> these pages by checking PageLRU because huge zero page isn't lru page as
> >>>> non-lru compound movable pages.
> >>>
> >>> It seems that memory_failure() also fails at get_any_page() with "hwpoison:
> >>> unhandlable page" message.
> >>>
> >>>   [16478.203474] page:00000000b6acdbd1 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x1810b4
> >>>   [16478.206612] flags: 0x57ffffc0801000(reserved|hwpoison|node=1|zone=2|lastcpupid=0x1fffff)
> >>>   [16478.209411] raw: 0057ffffc0801000 fffff11bc6042d08 fffff11bc6042d08 0000000000000000
> >>>   [16478.211921] raw: 0000000000000000 0000000000000000 00000001ffffffff 0000000000000000
> >>>   [16478.214473] page dumped because: hwpoison: unhandlable page
> >>>   [16478.216386] Memory failure: 0x1810b4: recovery action for unknown page: Ignored
> >>>
> >>> We can't handle errors on huge (or normal) zero page, so the current
> >>
> >> Sorry for confusing commit log again. I should have a coffee before I make this patch.
> >> Huge or normal zero page will fail at get_any_page because they're neither HWPoisonHandlable
> >> nor PageHuge.
> >>
> >>> behavior seems to me more suitable than "unsplit thp".
> >>>
> >>> Or if you have some producer to reach the following path with huge zero
> >>> page, could you share it?
> >>>
> >>
> >> What I mean is that non-lru movable compound page can reach here unexpected because __PageMovable(page)
> >> is handleable now. So get_any_page could succeed to grab the page refcnt. And since it's compound page,
> >> it will go through the split_huge_page_to_list because PageTransHuge checks PageHead(page) which can also
> >> be true for compound page. But this type of pages is unexpected for split_huge_page_to_list.
> >
> > Can we really handle non-LRU movable pages in memory failure
> > (uncorrectable errors)? Typically they are balloon, zsmalloc, etc.
> > Assuming we run into a base (4K) non-LRU movable page, we could reach
> > as far as identify_page_state(), it should not fall into any category
> > except me_unknown. So it seems we could just simply make it
> > unhandlable.
>
> There is the comment from memory_failure:
>         /*
>          * We ignore non-LRU pages for good reasons.
>          * - PG_locked is only well defined for LRU pages and a few others
>          * - to avoid races with __SetPageLocked()
>          * - to avoid races with __SetPageSlab*() (and more non-atomic ops)
>          * The check (unnecessarily) ignores LRU pages being isolated and
>          * walked by the page reclaim code, however that's not a big loss.
>          */
>
> So we could not handle non-LRU movable pages.
>
> What do you mean is something like below?
>
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index 5444a8ef4867..d80dbe0f20b6 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -1784,6 +1784,13 @@ int memory_failure(unsigned long pfn, int flags)
>                 }
>         }
>
> +       if (__PageMovable(hpage)) {
> +               put_page(p);
> +               action_result(pfn, MF_MSG_MOVALBE_PAGE, MF_IGNORED);
> +               res = -EBUSY;
> +               goto unlock_mutex;
> +       }
> +
>         if (PageTransHuge(hpage)) {
>                 /*
>                  * The flag must be set after the refcount is bumped
>
>
> i.e. Simply make non-LRU movable pages unhandlable ?

I'd prefer this personally. Something like the below (compile test only):

diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index 5444a8ef4867..789e40909ade 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -1176,12 +1176,18 @@ void ClearPageHWPoisonTakenOff(struct page *page)
  * does not return true for hugetlb or device memory pages, so it's assumed
  * to be called only in the context where we never have such pages.
  */
-static inline bool HWPoisonHandlable(struct page *page)
+static inline bool HWPoisonHandlable(struct page *page, unsigned long flags)
 {
- return PageLRU(page) || __PageMovable(page) || is_free_buddy_page(page);
+ bool movable = false;
+
+ /* Soft offline could mirgate non-LRU movable pages */
+ if ((flags & MF_SOFT_OFFLINE) && __PageMovable(page))
+ movable = true;
+
+ return movable || PageLRU(page) || is_free_buddy_page(page);
 }

-static int __get_hwpoison_page(struct page *page)
+static int __get_hwpoison_page(struct page *page, unsigned long flags)
 {
  struct page *head = compound_head(page);
  int ret = 0;
@@ -1196,7 +1202,7 @@ static int __get_hwpoison_page(struct page *page)
  * for any unsupported type of page in order to reduce the risk of
  * unexpected races caused by taking a page refcount.
  */
- if (!HWPoisonHandlable(head))
+ if (!HWPoisonHandlable(head, flags))
  return -EBUSY;

  if (get_page_unless_zero(head)) {
@@ -1221,7 +1227,7 @@ static int get_any_page(struct page *p, unsigned
long flags)

 try_again:
  if (!count_increased) {
- ret = __get_hwpoison_page(p);
+ ret = __get_hwpoison_page(p, flags);
  if (!ret) {
  if (page_count(p)) {
  /* We raced with an allocation, retry. */
@@ -1249,7 +1255,7 @@ static int get_any_page(struct page *p, unsigned
long flags)
  }
  }

- if (PageHuge(p) || HWPoisonHandlable(p)) {
+ if (PageHuge(p) || HWPoisonHandlable(p, flags)) {
  ret = 1;
  } else {
  /*

>
> >
> > But it should be handlable for soft-offline since it could be migrated.
> >
>
> Yes, non-LRU movable pages can be simply migrated.
>
> Many thanks.
>
> >
> >> Does this make sense for you? Thanks Naoya.
> >>
> >>> Thanks,
> >>> Naoya Horiguchi
> >>>
> >>>>
> >>>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> >>>> ---
> >>>>  mm/memory-failure.c | 14 ++++++++++++++
> >>>>  1 file changed, 14 insertions(+)
> >>>>
> >>>> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> >>>> index 23bfd809dc8c..ac6492e36978 100644
> >>>> --- a/mm/memory-failure.c
> >>>> +++ b/mm/memory-failure.c
> >>>> @@ -1792,6 +1792,20 @@ int memory_failure(unsigned long pfn, int flags)
> >>>>      }
> >>>>
> >>>>      if (PageTransHuge(hpage)) {
> >>>> +            /*
> >>>> +             * The non-lru compound movable pages could be taken for
> >>>> +             * transhuge pages. Also huge zero page could reach here
> >>>> +             * and if we ever try to split it, the VM_BUG_ON_PAGE will
> >>>> +             * be triggered in split_huge_page_to_list(). Skip these
> >>>> +             * pages by checking PageLRU because huge zero page isn't
> >>>> +             * lru page as non-lru compound movable pages.
> >>>> +             */
> >>>> +            if (!PageLRU(hpage)) {
> >>>> +                    put_page(p);
> >>>> +                    action_result(pfn, MF_MSG_UNSPLIT_THP, MF_IGNORED);
> >>>> +                    res = -EBUSY;
> >>>> +                    goto unlock_mutex;
> >>>> +            }
> >>>>              /*
> >>>>               * The flag must be set after the refcount is bumped
> >>>>               * otherwise it may race with THP split.
> >>>> --
> >>>> 2.23.0
> >>
> >>
> > .
> >
>