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

[Zi Yan]

On 21 Oct 2025, at 11:44, David Hildenbrand wrote:

> On 21.10.25 03:23, Zi Yan wrote:
>> On 20 Oct 2025, at 19:41, Yang Shi wrote:
>>
>>> On Mon, Oct 20, 2025 at 12:46 PM Zi Yan <ziy@xxxxxxxxxx> wrote:
>>>>
>>>> On 17 Oct 2025, at 15:11, Yang Shi wrote:
>>>>
>>>>> On Wed, Oct 15, 2025 at 8:38 PM Zi Yan <ziy@xxxxxxxxxx> 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@xxxxxxxxxx>
>>>>>> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
>>>>>> Reviewed-by: Luis Chamberlain <mcgrof@xxxxxxxxxx>
>>>>>> ---
>>>>>>   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.
>>>
>>> I'm a little bit confused. Didn't this patch split large folio to
>>> new-order-large-folio (new order is min order)? So this patch had
>>> code:
>>> if (try_to_split_thp_page(p, new_order, false) || new_order) {
>>
>> Yes, but this is Patch 2 in this series. Patch 1 is
>> "mm/huge_memory: do not change split_huge_page*() target order silently."
>> and sent separately as a hotfix[1].
>
> I'm confused now as well. I'd like to review, will there be a v3 that only contains patch #2+#3?

Yes. The new V3 will have 3 patches:
1. a new patch addresses Yang’s concern on setting has_hwpoisoned on after-split
large folios.
2. patch#2,
3. patch#3.

The plan is to send them out once patch 1 is upstreamed. Let me know if you think
it is OK to send them out earlier as Andrew already picked up patch 1.

I also would like to get some feedback on my approach to setting has_hwpoisoned:

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.

Best Regards,
Yan, Zi