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

[Miaohe Lin <linmiaohe@huawei.com>]

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?

>   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()?

> +
> +	/*
> +	 * 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().

> +		SetPageHWPoison(curr->page);

If hugetlb folio vmemmap is optimized, I think SetPageHWPoison might trigger BUG.

> +		kfree(curr);
> +	}

Above logic is almost same as folio_clear_hugetlb_hwpoison. Maybe we can reuse that?

> +
> +	/* Refcount now should be zero and ready to dissolve folio. */
> +	ret = dissolve_free_hugetlb_folio(folio);
> +	if (ret)
> +		pr_err("failed to dissolve hugetlb folio: %d\n", ret);
> +}
> +

Thanks.
.