Message-ID: <3AEF55E9-5E50-4F4F-92B8-EA9955296BFA@nvidia.com>
Date: Mon, 20 Oct 2025 16:09:33 -0400
From: Zi Yan <ziy@...dia.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: linmiaohe@...wei.com, david@...hat.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,
 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 17 Oct 2025, at 5:33, Lorenzo Stoakes wrote:

> On Wed, Oct 15, 2025 at 11:34:51PM -0400, Zi Yan 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);
>
> I wonder if we need a wrapper for these list==NULL cases, as
> split_huge_page_to_list_to_order suggests you always have a list provided... and
> this is ugly :)
>
> split_huge_page_to_order() seems good.

Yes, this suggestion motivated me to remove unused list==NULL parameter in
try_folio_split_to_order(). Thanks.

>
>>  	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);
>
> Newline after decl?

Sure.

>
>>  		/*
>>  		 * 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) {
>
> Please use /* release= */false here

OK.

>
>
> I'm also not sure about the logic here, it feels unclear.
>
> Something like:
>
> 	err = try_to_to_split_thp_page(p, new_order, /* release= */false);
>
> 		/*
> 		 * If the folio cannot be split, kill the process.
> 		 * If it can be split, but not to order-0, then this defeats the
> 		 * expectation that we do so, but we want the split to have been
> 		 * made to
> 		 */
>
> 	if (err || new_order > 0) {
> 	}

Will make the change.

>
>
>> +			/* 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
>
> Typo perferred -> preferred.

Got it.

>
>
>> +		 * is done in memory_failure().
>
> I'm confused as to your comment here though, we're not splitting it like
> memory_failure()? We're splitting a. with release and b. only if we can target
> order-0.
>
> So how would this preference in any way be a thing that happens? :) I may be
> missing something here.

For non LBS folios, min_order_for_split() returns 0. In that case, the split
would happen.

>
>> +		 */
>> +		if (new_order || try_to_split_thp_page(page, new_order, true)) {
>
> Same comment as above with /* release= */true.

Sure.

>
> You should pass 0 not new_order to try_to_split_thp_page() here as it has to be
> 0 for the function to be invoked and that's just obviously clearer.

OK. How about try_to_split_thp_page(page, /* new_order= */ 0, /* release= */ true)?
So that readers can tell 0 is the value of new_order.

>
>
>>  			pr_info("%#lx: thp split failed\n", pfn);
>>  			return -EBUSY;
>>  		}

Thank you for the feedback.


--
Best Regards,
Yan, Zi

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