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

From: Yang Shi
Date: Thu Mar 10 2022 - 14:32:26 EST

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On Thu, Mar 10, 2022 at 3:46 AM Miaohe Lin <linmiaohe@xxxxxxxxxx> wrote:
>
> On 2022/3/9 2:47, Yang Shi wrote:
> > On Tue, Mar 8, 2022 at 4:36 AM Miaohe Lin <linmiaohe@xxxxxxxxxx> wrote:
> >>
> >> On 2022/3/8 3:53, Yang Shi wrote:
> >>> On Sun, Mar 6, 2022 at 11:07 PM Miaohe Lin <linmiaohe@xxxxxxxxxx> 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 think about the below code more carefully and I found that this will make
> hwpoison_filter can't handle the non-LRU movable pages now. Because non-LRU
> movable pages return early now and thus can't reach the hwpoison_filter. This
> results in a inconsistent behavior with previous one. So I think the origin
> fixup of this patch is more suitable. What do you think?

I'm not familiar with hwpoison_filter(), it seems like a test helper
for error injection. I don't think hwpoison_filter() is used to filter
unhandlable page, for example, slab page, IIUC. So the non-LRU movable
pages should be treated the same. If so, the old behavior was simply
wrong.

>
> Thanks.
>
> > 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@xxxxxxxxxx>
> >>>>>> ---
> >>>>>> 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
> >>>>
> >>>>
> >>> .
> >>>
> >>
> > .
> >
>
>

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