Re: [PATCH v3 1/3] mm: memfd/hugetlb: introduce memfd-based userspace MFR policy

[Jiaqi Yan <jiaqiyan@google.com>]

On Mon, Mar 9, 2026 at 12:41 AM Miaohe Lin <linmiaohe@huawei.com> wrote:
>
> On 2026/3/9 12:53, Jiaqi Yan wrote:
> > On Mon, Feb 23, 2026 at 11:30 PM Miaohe Lin <linmiaohe@huawei.com> wrote:
> >>
> >> On 2026/2/13 13:01, Jiaqi Yan wrote:
> >>> On Mon, Feb 9, 2026 at 11:31 PM Miaohe Lin <linmiaohe@huawei.com> wrote:
> >>>>
> >>>> On 2026/2/10 12:47, Jiaqi Yan wrote:
> >>>>> On Mon, Feb 9, 2026 at 3:54 AM Miaohe Lin <linmiaohe@huawei.com> wrote:
> >>>>>>
> >>>>>> On 2026/2/4 3:23, Jiaqi Yan wrote:
> >>>>>>> Sometimes immediately hard offlining a large chunk of contigous memory
> >>>>>>> having uncorrected memory errors (UE) may not be the best option.
> >>>>>>> Cloud providers usually serve capacity- and performance-critical guest
> >>>>>>> memory with 1G HugeTLB hugepages, as this significantly reduces the
> >>>>>>> overhead associated with managing page tables and TLB misses. However,
> >>>>>>> for today's HugeTLB system, once a byte of memory in a hugepage is
> >>>>>>> hardware corrupted, the kernel discards the whole hugepage, including
> >>>>>>> the healthy portion. Customer workload running in the VM can hardly
> >>>>>>> recover from such a great loss of memory.
> >>>>>>
> >>>>>> Thanks for your patch. Some questions below.
> >>>>>>
> >>>>>>>
> >>>>>>> Therefore keeping or discarding a large chunk of contiguous memory
> >>>>>>> owned by userspace (particularly to serve guest memory) due to
> >>>>>>> recoverable UE may better be controlled by userspace process
> >>>>>>> that owns the memory, e.g. VMM in the Cloud environment.
> >>>>>>>
> >>>>>>> Introduce a memfd-based userspace memory failure (MFR) policy,
> >>>>>>> MFD_MF_KEEP_UE_MAPPED. It is possible to support for other memfd,
> >>>>>>> but the current implementation only covers HugeTLB.
> >>>>>>>
> >>>>>>> For a hugepage associated with MFD_MF_KEEP_UE_MAPPED enabled memfd,
> >>>>>>> whenever it runs into a new UE,
> >>>>>>>
> >>>>>>> * MFR defers hard offline operations, i.e., unmapping and
> >>>>>>
> >>>>>> So the folio can't be unpoisoned until hugetlb folio becomes free?
> >>>>>
> >>>>> Are you asking from testing perspective, are we still able to clean up
> >>>>> injected test errors via unpoison_memory() with MFD_MF_KEEP_UE_MAPPED?
> >>>>>
> >>>>> If so, unpoison_memory() can't turn the HWPoison hugetlb page to
> >>>>> normal hugetlb page as MFD_MF_KEEP_UE_MAPPED automatically dissolves
> >>>>
> >>>> We might loss some testability but that should be an acceptable compromise.
> >>>
> >>> To clarify, looking at unpoison_memory(), it seems unpoison should
> >>> still work if called before truncated or memfd closed.
> >>>
> >>> What I wanted to say is, for my test hugetlb-mfr.c, since I really
> >>> want to test the cleanup code (dissolving free hugepage having
> >>> multiple errors) after truncation or memfd closed, so we can only
> >>> unpoison the raw pages rejected by buddy allocator.
> >>>
> >>>>
> >>>>> it. unpoison_memory(pfn) can probably still turn the HWPoison raw page
> >>>>> back to a normal one, but you already lost the hugetlb page.
> >>>>>
> >>>>>>
> >>>>>>>   dissolving. MFR still sets HWPoison flag, holds a refcount
> >>>>>>>   for every raw HWPoison page, record them in a list, sends SIGBUS
> >>>>>>>   to the consuming thread, but si_addr_lsb is reduced to PAGE_SHIFT.
> >>>>>>>   If userspace is able to handle the SIGBUS, the HWPoison hugepage
> >>>>>>>   remains accessible via the mapping created with that memfd.
> >>>>>>>
> >>>>>>> * If the memory was not faulted in yet, the fault handler also
> >>>>>>>   allows fault in the HWPoison folio.
> >>>>>>>
> >>>>>>> For a MFD_MF_KEEP_UE_MAPPED enabled memfd, when it is closed, or
> >>>>>>> when userspace process truncates its hugepages:
> >>>>>>>
> >>>>>>> * When the HugeTLB in-memory file system removes the filemap's
> >>>>>>>   folios one by one, it asks MFR to deal with HWPoison folios
> >>>>>>>   on the fly, implemented by filemap_offline_hwpoison_folio().
> >>>>>>>
> >>>>>>> * MFR drops the refcounts being held for the raw HWPoison
> >>>>>>>   pages within the folio. Now that the HWPoison folio becomes
> >>>>>>>   free, MFR dissolves it into a set of raw pages. The healthy pages
> >>>>>>>   are recycled into buddy allocator, while the HWPoison ones are
> >>>>>>>   prevented from re-allocation.
> >>>>>>>
> >>>>>> ...
> >>>>>>
> >>>>>>>
> >>>>>>> +static void filemap_offline_hwpoison_folio_hugetlb(struct folio *folio)
> >>>>>>> +{
> >>>>>>> +     int ret;
> >>>>>>> +     struct llist_node *head;
> >>>>>>> +     struct raw_hwp_page *curr, *next;
> >>>>>>> +
> >>>>>>> +     /*
> >>>>>>> +      * Since folio is still in the folio_batch, drop the refcount
> >>>>>>> +      * elevated by filemap_get_folios.
> >>>>>>> +      */
> >>>>>>> +     folio_put_refs(folio, 1);
> >>>>>>> +     head = llist_del_all(raw_hwp_list_head(folio));
> >>>>>>
> >>>>>> We might race with get_huge_page_for_hwpoison()? llist_add() might be called
> >>>>>> by folio_set_hugetlb_hwpoison() just after llist_del_all()?
> >>>>>
> >>>>> Oh, when there is a new UE while we releasing the folio here, right?
> >>>>
> >>>> Right.
> >>>>
> >>>>> In that case, would mutex_lock(&mf_mutex) eliminate potential race?
> >>>>
> >>>> IMO spin_lock_irq(&hugetlb_lock) might be better.
> >>>
> >>> Looks like I don't need any lock given the correction below.
> >>>
> >>>>
> >>>>>
> >>>>>>
> >>>>>>> +
> >>>>>>> +     /*
> >>>>>>> +      * Release refcounts held by try_memory_failure_hugetlb, one per
> >>>>>>> +      * HWPoison-ed page in the raw hwp list.
> >>>>>>> +      *
> >>>>>>> +      * Set HWPoison flag on each page so that free_has_hwpoisoned()
> >>>>>>> +      * can exclude them during dissolve_free_hugetlb_folio().
> >>>>>>> +      */
> >>>>>>> +     llist_for_each_entry_safe(curr, next, head, node) {
> >>>>>>> +             folio_put(folio);
> >>>>>>
> >>>>>> The hugetlb folio refcnt will only be increased once even if it contains multiple UE sub-pages.
> >>>>>> See __get_huge_page_for_hwpoison() for details. So folio_put() might be called more times than
> >>>>>> folio_try_get() in __get_huge_page_for_hwpoison().
> >>>>>
> >>>>> The changes in folio_set_hugetlb_hwpoison() should make
> >>>>> __get_huge_page_for_hwpoison() not to take the "out" path which
> >>>>> decrease the increased refcount for folio. IOW, every time a new UE
> >>>>> happens, we handle the hugetlb page as if it is an in-use hugetlb
> >>>>> page.
> >>>>
> >>>> See below code snippet (comment [1] and [2]):
> >>>>
> >>>> int __get_huge_page_for_hwpoison(unsigned long pfn, int flags,
> >>>>                                  bool *migratable_cleared)
> >>>> {
> >>>>         struct page *page = pfn_to_page(pfn);
> >>>>         struct folio *folio = page_folio(page);
> >>>>         int ret = 2;    /* fallback to normal page handling */
> >>>>         bool count_increased = false;
> >>>>
> >>>>         if (!folio_test_hugetlb(folio))
> >>>>                 goto out;
> >>>>
> >>>>         if (flags & MF_COUNT_INCREASED) {
> >>>>                 ret = 1;
> >>>>                 count_increased = true;
> >>>>         } else if (folio_test_hugetlb_freed(folio)) {
> >>>>                 ret = 0;
> >>>>         } else if (folio_test_hugetlb_migratable(folio)) {
> >>>>
> >>>>                    ^^^^*hugetlb_migratable is checked before trying to get folio refcnt* [1]
> >>>>
> >>>>                 ret = folio_try_get(folio);
> >>>>                 if (ret)
> >>>>                         count_increased = true;
> >>>>         } else {
> >>>>                 ret = -EBUSY;
> >>>>                 if (!(flags & MF_NO_RETRY))
> >>>>                         goto out;
> >>>>         }
> >>>>
> >>>>         if (folio_set_hugetlb_hwpoison(folio, page)) {
> >>>>                 ret = -EHWPOISON;
> >>>>                 goto out;
> >>>>         }
> >>>>
> >>>>         /*
> >>>>          * Clearing hugetlb_migratable for hwpoisoned hugepages to prevent them
> >>>>          * from being migrated by memory hotremove.
> >>>>          */
> >>>>         if (count_increased && folio_test_hugetlb_migratable(folio)) {
> >>>>                 folio_clear_hugetlb_migratable(folio);
> >>>>
> >>>>                 ^^^^^*hugetlb_migratable is cleared when first time seeing folio* [2]
> >>>>
> >>>>                 *migratable_cleared = true;
> >>>>         }
> >>>>
> >>>> Or am I miss something?
> >>>
> >>> Thanks for your explaination! You are absolutely right. It turns out
> >>> the extra refcount I saw (during running hugetlb-mfr.c) on the folio
> >>> at the moment of filemap_offline_hwpoison_folio_hugetlb() is actually
> >>> because of the MF_COUNT_INCREASED during MADV_HWPOISON. In the past I
> >>> used to think that is the effect of folio_try_get() in
> >>> __get_huge_page_for_hwpoison(), and it is wrong. Now I see two cases:
> >>> - MADV_HWPOISON: instead of __get_huge_page_for_hwpoison(),
> >>> madvise_inject_error() is the one that increments hugepage refcount
> >>> for every error injected. Different from other cases,
> >>> MFD_MF_KEEP_UE_MAPPED makes the hugepage still a in-use page after
> >>> memory_failure(MF_COUNT_INCREASED), so I think madvise_inject_error()
> >>> should decrement in MFD_MF_KEEP_UE_MAPPED case.
> >>> - In the real world: as you pointed out, MF always just increments
> >>> hugepage refcount once in __get_huge_page_for_hwpoison(), even if it
> >>> runs into multiple errors. When
> >>
> >> This might not always hold true. When MF occurs while hugetlb folio is under isolation(hugetlb_migratable is
> >> cleared and extra folio refcnt is held by isolating code in that case), __get_huge_page_for_hwpoison won't get
> >> extra folio refcnt.
> >>
> >>> filemap_offline_hwpoison_folio_hugetlb() drops the refcount elevated
> >>> by filemap_get_folios(), it only needs to decrement again if
> >>> folio_ref_dec_and_test() returns false. I tested something like below:
> >>>
> >>>     /* drop the refcount elevated by filemap_get_folios. */
> >>>     folio_put(folio);
> >>>     if (folio_ref_count(folio))
> >>>         folio_put(folio);
> >>>     /* now refcount should be zero. */
> >>>     ret = dissolve_free_hugetlb_folio(folio);
> >>
> >> So I think above code might drop the folio refcnt held by isolating code.
> >
> > Hi Miaohe, thanks for raising the concern. Given two things below
> > - both folio_isolate_hugetlb() and get_huge_page_for_hwpoison() are
> > guarded by hugetlb_lock.
> > - hugetlb_update_hwpoison() only folio_test_set_hwpoison() for
> > non-isolated folio after folio_try_get() succeeds.
> >
> > as long as folio_test_set_hwpoison() is true here, this refcount
> > should never come from folio_isolate_hugetlb(). What do you think?
> >
>
> Let's think about below scenario. When __get_huge_page_for_hwpoison() encounters an
> isolated hugetlb folio:
>
> int __get_huge_page_for_hwpoison(unsigned long pfn, int flags,
>                                  bool *migratable_cleared)
> {
>         struct page *page = pfn_to_page(pfn);
>         struct folio *folio = page_folio(page);
>         bool count_increased = false;
>         int ret, rc;
>
>         if (!folio_test_hugetlb(folio)) {
>                 ret = MF_HUGETLB_NON_HUGEPAGE;
>                 goto out;
>         } else if (flags & MF_COUNT_INCREASED) {
>                 ret = MF_HUGETLB_IN_USED;
>                 count_increased = true;
>         } else if (folio_test_hugetlb_freed(folio)) {
>                 ret = MF_HUGETLB_FREED;
>         } else if (folio_test_hugetlb_migratable(folio)) {
>
>                    ^^^^*Since hugetlb_migratable is cleared for the isolated hugetlb folio*
>
>                 if (folio_try_get(folio)) {
>                         ret = MF_HUGETLB_IN_USED;
>                         count_increased = true;
>                 } else {
>                         ret = MF_HUGETLB_FREED;
>                 }
>         } else {
>
>                   ^^^^*Code will reach here without extra refcnt increased*
>
>                 ret = MF_HUGETLB_RETRY;
>                 if (!(flags & MF_NO_RETRY))
>                         goto out;
>         }
>
>         *Code will reach here after retry*

You are right, thanks for pointing that out. Let me think about more
how to handle this.

>         rc = hugetlb_update_hwpoison(folio, page);
>         if (rc >= MF_HUGETLB_FOLIO_PRE_POISONED) {
>                 ret = rc;
>                 goto out;
>         }
>
> So hugetlb_update_hwpoison() will be called even for folio under isolation
> without folio_try_get(). Or am I miss something?

Just a random question: if MF never increments a hugepage's refcount,
what does the folio_put() in me_huge_page() (when mapping = null) do?
Is it dropping for something other than MF?

>
> Thanks.
> .