Re: [PATCH v2 2/6] mm/gup: local lru_add_drain() to avoid lru_add_drain_all()

[David Hildenbrand <david@redhat.com>]

On 09.09.25 12:52, Kiryl Shutsemau wrote:
> On Tue, Sep 09, 2025 at 09:56:30AM +0200, David Hildenbrand wrote:
>> On 09.09.25 00:16, Hugh Dickins wrote:
>>> In many cases, if collect_longterm_unpinnable_folios() does need to
>>> drain the LRU cache to release a reference, the cache in question is
>>> on this same CPU, and much more efficiently drained by a preliminary
>>> local lru_add_drain(), than the later cross-CPU lru_add_drain_all().
>>>
>>> Marked for stable, to counter the increase in lru_add_drain_all()s
>>> from "mm/gup: check ref_count instead of lru before migration".
>>> Note for clean backports: can take 6.16 commit a03db236aebf ("gup:
>>> optimize longterm pin_user_pages() for large folio") first.
>>>
>>> Signed-off-by: Hugh Dickins <hughd@google.com>
>>> Cc: <stable@vger.kernel.org>
>>> ---
>>>    mm/gup.c | 15 +++++++++++----
>>>    1 file changed, 11 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/mm/gup.c b/mm/gup.c
>>> index 82aec6443c0a..b47066a54f52 100644
>>> --- a/mm/gup.c
>>> +++ b/mm/gup.c
>>> @@ -2287,8 +2287,8 @@ static unsigned long collect_longterm_unpinnable_folios(
>>>    		struct pages_or_folios *pofs)
>>>    {
>>>    	unsigned long collected = 0;
>>> -	bool drain_allow = true;
>>>    	struct folio *folio;
>>> +	int drained = 0;
>>>    	long i = 0;
>>>    	for (folio = pofs_get_folio(pofs, i); folio;
>>> @@ -2307,10 +2307,17 @@ static unsigned long collect_longterm_unpinnable_folios(
>>>    			continue;
>>>    		}
>>> -		if (drain_allow && folio_ref_count(folio) !=
>>> -				   folio_expected_ref_count(folio) + 1) {
>>> +		if (drained == 0 &&
>>> +				folio_ref_count(folio) !=
>>> +				folio_expected_ref_count(folio) + 1) {
>>
>> I would just have indented this as follows:
>>
>> 		if (drained == 0 &&
>> 		    folio_ref_count(folio) != folio_expected_ref_count(folio) + 1) {
> 
> Do we want folio_check_expected_ref_count(folio, offset)?

Not sure, if so outside of this patch series to also cover the other 
handful of cases.

	folio_has_unexpected_refs(folio, offset)

Would probably be clearer.

-- 
Cheers

David / dhildenb