Re: [PATCH v2 2/3] mm/memory-failure: improve large block size folio handling.

[Zi Yan <ziy@nvidia.com>]

On 17 Oct 2025, at 15:11, Yang Shi wrote:

> On Wed, Oct 15, 2025 at 8:38 PM Zi Yan <ziy@nvidia.com> wrote:
>>
>> Large block size (LBS) folios cannot be split to order-0 folios but
>> min_order_for_folio(). Current split fails directly, but that is not
>> optimal. Split the folio to min_order_for_folio(), so that, after split,
>> only the folio containing the poisoned page becomes unusable instead.
>>
>> For soft offline, do not split the large folio if it cannot be split to
>> order-0. Since the folio is still accessible from userspace and premature
>> split might lead to potential performance loss.
>>
>> Suggested-by: Jane Chu <jane.chu@oracle.com>
>> Signed-off-by: Zi Yan <ziy@nvidia.com>
>> Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
>> ---
>>  mm/memory-failure.c | 25 +++++++++++++++++++++----
>>  1 file changed, 21 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
>> index f698df156bf8..443df9581c24 100644
>> --- a/mm/memory-failure.c
>> +++ b/mm/memory-failure.c
>> @@ -1656,12 +1656,13 @@ static int identify_page_state(unsigned long pfn, struct page *p,
>>   * there is still more to do, hence the page refcount we took earlier
>>   * is still needed.
>>   */
>> -static int try_to_split_thp_page(struct page *page, bool release)
>> +static int try_to_split_thp_page(struct page *page, unsigned int new_order,
>> +               bool release)
>>  {
>>         int ret;
>>
>>         lock_page(page);
>> -       ret = split_huge_page(page);
>> +       ret = split_huge_page_to_list_to_order(page, NULL, new_order);
>>         unlock_page(page);
>>
>>         if (ret && release)
>> @@ -2280,6 +2281,7 @@ int memory_failure(unsigned long pfn, int flags)
>>         folio_unlock(folio);
>>
>>         if (folio_test_large(folio)) {
>> +               int new_order = min_order_for_split(folio);
>>                 /*
>>                  * The flag must be set after the refcount is bumped
>>                  * otherwise it may race with THP split.
>> @@ -2294,7 +2296,14 @@ int memory_failure(unsigned long pfn, int flags)
>>                  * page is a valid handlable page.
>>                  */
>>                 folio_set_has_hwpoisoned(folio);
>> -               if (try_to_split_thp_page(p, false) < 0) {
>> +               /*
>> +                * If the folio cannot be split to order-0, kill the process,
>> +                * but split the folio anyway to minimize the amount of unusable
>> +                * pages.
>> +                */
>> +               if (try_to_split_thp_page(p, new_order, false) || new_order) {
>
> folio split will clear PG_has_hwpoisoned flag. It is ok for splitting
> to order-0 folios because the PG_hwpoisoned flag is set on the
> poisoned page. But if you split the folio to some smaller order large
> folios, it seems you need to keep PG_has_hwpoisoned flag on the
> poisoned folio.

OK, this means all pages in a folio with folio_test_has_hwpoisoned() should be
checked to be able to set after-split folio's flag properly. Current folio
split code does not do that. I am thinking about whether that causes any
issue. Probably not, because:

1. before Patch 1 is applied, large after-split folios are already causing
a warning in memory_failure(). That kinda masks this issue.
2. after Patch 1 is applied, no large after-split folios will appear,
since the split will fail.

@Miaohe and @Jane, please let me know if my above reasoning makes sense or not.

To make this patch right, folio's has_hwpoisoned flag needs to be preserved
like what Yang described above. My current plan is to move
folio_clear_has_hwpoisoned(folio) into __split_folio_to_order() and
scan every page in the folio if the folio's has_hwpoisoned is set.
There will be redundant scans in non uniform split case, since a has_hwpoisoned
folio can be split multiple times (leading to multiple page scans), unless
the scan result is stored.

@Miaohe and @Jane, is it possible to have multiple HW poisoned pages in
a folio? Is the memory failure process like 1) page access causing MCE,
2) memory_failure() is used to handle it and split the large folio containing
it? Or multiple MCEs can be received and multiple pages in a folio are marked
then a split would happen?

>
> Yang
>
>
>> +                       /* get folio again in case the original one is split */
>> +                       folio = page_folio(p);
>>                         res = -EHWPOISON;
>>                         kill_procs_now(p, pfn, flags, folio);
>>                         put_page(p);
>> @@ -2621,7 +2630,15 @@ static int soft_offline_in_use_page(struct page *page)
>>         };
>>
>>         if (!huge && folio_test_large(folio)) {
>> -               if (try_to_split_thp_page(page, true)) {
>> +               int new_order = min_order_for_split(folio);
>> +
>> +               /*
>> +                * If the folio cannot be split to order-0, do not split it at
>> +                * all to retain the still accessible large folio.
>> +                * NOTE: if getting free memory is perferred, split it like it
>> +                * is done in memory_failure().
>> +                */
>> +               if (new_order || try_to_split_thp_page(page, new_order, true)) {
>>                         pr_info("%#lx: thp split failed\n", pfn);
>>                         return -EBUSY;
>>                 }
>> --
>> 2.51.0
>>
>>


--
Best Regards,
Yan, Zi