linux-kernel - Re: [PATCH v2 4/4] mm/huge_memory: fix folio split stats counting

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Message-ID: <539E2FBF-C5B4-4B2C-9AE0-A1F3A9203A1D@nvidia.com>
Date: Mon, 24 Nov 2025 12:23:31 -0500
From: Zi Yan <ziy@...dia.com>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 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>,
 Miaohe Lin <linmiaohe@...wei.com>, Naoya Horiguchi <nao.horiguchi@...il.com>,
 Wei Yang <richard.weiyang@...il.com>, Balbir Singh <balbirs@...dia.com>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/4] mm/huge_memory: fix folio split stats counting

On 24 Nov 2025, at 5:45, David Hildenbrand (Red Hat) wrote:

> On 11/22/25 03:55, Zi Yan wrote:
>> The "return <error code>" statements for error checks at the beginning of
>> __folio_split() skip necessary count_vm_event() and count_mthp_stat() at
>> the end of the function. Fix these by replacing them with
>> "ret = <error code>; goto out;".
>>
>> Signed-off-by: Zi Yan <ziy@...dia.com>
>> ---
>>   mm/huge_memory.c | 14 +++++++++-----
>>   1 file changed, 9 insertions(+), 5 deletions(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index ebc3ba0907fd..a42c4f29ce4f 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -3954,16 +3954,20 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>>   	VM_WARN_ON_ONCE_FOLIO(!folio_test_locked(folio), folio);
>>   	VM_WARN_ON_ONCE_FOLIO(!folio_test_large(folio), folio);
>>  -	if (folio != page_folio(split_at) || folio != page_folio(lock_at))
>> -		return -EINVAL;
>> +	if (folio != page_folio(split_at) || folio != page_folio(lock_at)) {
>> +		ret = -EINVAL;
>> +		goto out;
>> +	}
>>  -	if (new_order >= old_order)
>> -		return -EINVAL;
>> +	if (new_order >= old_order) {
>> +		ret = -EINVAL;
>> +		goto out;
>> +	}
>>    	ret = folio_check_splittable(folio, new_order, split_type,
>>   				     /* warn = */ true);
>>   	if (ret)
>> -		return ret;
>> +		goto out;
>>    	if (is_anon) {
>>   		/*
>
> I guess this is not Fixes:/stable material. Wonder if such early (mostly -EINVAL etc) checks were at some point not intended to be counted.

I do not think it is worth Fixes/stable, since most checks should be caught
during development and not be triggered, except folio_test_writeback(folio)
one. And no one complained so far.

The inconsistency starts from commit 59807685a7e7 ("mm, THP, swap: support
splitting THP for THP swap out”), where if (PageWriteback(page)) return -EBUSY;
was added. Then commit 478d134e9506 ("mm/huge_memory: do not overkill when
splitting huge_zero_page") followed and so on.

This patch is intended to make code consistent.

>
> In any case
>
> Acked-by: David Hildenbrand (Red Hat) <david@...nel.org>

Thanks.

Best Regards,
Yan, Zi