Re: [PATCH 2/2] mm: avoid unnecessary PTE table lock during initial swap folio scan

[Wei Yang <richard.weiyang@gmail.com>]

On Fri, Dec 19, 2025 at 09:47:17AM +0100, David Hildenbrand (Red Hat) wrote:
>On 12/16/25 08:59, Wei Yang wrote:
>> The alloc_swap_folio() function performs an initial scan of the PTE
>> table solely to determine the potential size (order) of the folio
>> content that needs to be swapped in.
>> 
>> Locking the PTE table during this initial read is unnecessary for two
>> reasons:
>> 
>>    * We are not writing to the PTE table at this stage.
>> 
>>    * The code will re-check and lock the table again immediately before
>>      any actual modification is attempted.
>> 
>> This commit refactors the initial scan to map the PTE table without
>> acquiring the lock. This reduces contention and overhead, improving
>> performance of the swap-in path.
>> 
>> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
>> Cc: Chuanhua Han <hanchuanhua@oppo.com>
>> Cc: Barry Song <v-songbaohua@oppo.com>
>> ---
>>   mm/memory.c | 6 ++----
>>   1 file changed, 2 insertions(+), 4 deletions(-)
>> 
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 1b8ef4f0ea60..f8d6adfa83d7 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -4529,7 +4529,6 @@ static struct folio *alloc_swap_folio(struct vm_fault *vmf)
>>   	struct folio *folio;
>>   	unsigned long addr;
>>   	softleaf_t entry;
>> -	spinlock_t *ptl;
>>   	pte_t *pte;
>>   	gfp_t gfp;
>>   	int order;
>> @@ -4563,8 +4562,7 @@ static struct folio *alloc_swap_folio(struct vm_fault *vmf)
>>   	if (!orders)
>>   		goto fallback;
>> -	pte = pte_offset_map_lock(vmf->vma->vm_mm, vmf->pmd,
>> -				  vmf->address & PMD_MASK, &ptl);
>> +	pte = pte_offset_map(vmf->pmd, vmf->address & PMD_MASK);
>>   	if (unlikely(!pte))
>
>Can can_swapin_thp() deal with concurrent unmap and possible freeing of
>pages+swap?
>
>We have some code that depends on swap entries stabilizing the swap device
>etc; the moment you allow for that concurrently to go away you open a can of
>worns.
>

Sorry I don't follow you.

You mean some swap entry would be unmapped and cleared?

This is the first scan of swap entries, and there is another
swap_entry_batch() before real work with ptl locked to make sure the range is
still valid for swap in.

If someone change the pte at this moment, it will be detected after ptl
locked by swap_entry_batch().

Maybe I misunderstand you. Would you mind giving me more hint?

>-- 
>Cheers
>
>David

-- 
Wei Yang
Help you, Help me