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

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Message-ID: <b353587b-ef50-41ab-8dd2-93330098053e@redhat.com>
Date: Tue, 21 Oct 2025 17:44:08 +0200
From: David Hildenbrand <david@...hat.com>
To: Zi Yan <ziy@...dia.com>, Yang Shi <shy828301@...il.com>
Cc: linmiaohe@...wei.com, jane.chu@...cle.com, kernel@...kajraghav.com,
 syzbot+e6367ea2fdab6ed46056@...kaller.appspotmail.com,
 syzkaller-bugs@...glegroups.com, akpm@...ux-foundation.org,
 mcgrof@...nel.org, nao.horiguchi@...il.com,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, Nico Pache <npache@...hat.com>,
 Ryan Roberts <ryan.roberts@....com>, Dev Jain <dev.jain@....com>,
 Barry Song <baohua@...nel.org>, Lance Yang <lance.yang@...ux.dev>,
 "Matthew Wilcox (Oracle)" <willy@...radead.org>,
 Wei Yang <richard.weiyang@...il.com>, linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2 2/3] mm/memory-failure: improve large block size folio
 handling.

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@...dia.com> wrote:
>>>
>>> On 17 Oct 2025, at 15:11, Yang Shi wrote:
>>>
>>>> On Wed, Oct 15, 2025 at 8:38 PM Zi Yan <ziy@...dia.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@...cle.com>
>>>>> Signed-off-by: Zi Yan <ziy@...dia.com>
>>>>> Reviewed-by: Luis Chamberlain <mcgrof@...nel.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.
>>
>> 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?

Thanks!

-- 
Cheers

David / dhildenb