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

[Miaohe Lin <linmiaohe@huawei.com>]

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.

Thanks.
.