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

[Jiaqi Yan <jiaqiyan@google.com>]

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?

For folio under isolation, MF ignores it without
folio_test_set_hwpoison(), and
filemap_offline_hwpoison_folio_hugetlb() won't happen at all.
For HWPoison folio, MF has made the folio no longer being able to
isolate/migrate.

>
> Thanks.
> .