Re: [PATCH 6/7] mm: folio_may_be_cached() unless folio_test_large()

[David Hildenbrand <david@redhat.com>]

On 08.09.25 13:19, Hugh Dickins wrote:
> On Mon, 1 Sep 2025, David Hildenbrand wrote:
>> On 31.08.25 11:16, Hugh Dickins wrote:
>>> mm/swap.c and mm/mlock.c agree to drain any per-CPU batch as soon as
>>> a large folio is added: so collect_longterm_unpinnable_folios() just
>>> wastes effort when calling lru_add_drain_all() on a large folio.
>>>
>>> But although there is good reason not to batch up PMD-sized folios,
>>> we might well benefit from batching a small number of low-order mTHPs
>>> (though unclear how that "small number" limitation will be implemented).
>>>
>>> So ask if folio_may_be_cached() rather than !folio_test_large(), to
>>> insulate those particular checks from future change.  Name preferred
>>> to "folio_is_batchable" because large folios can well be put on a batch:
>>> it's just the per-CPU LRU caches, drained much later, which need care.
>>>
>>> Marked for stable, to counter the increase in lru_add_drain_all()s
>>> from "mm/gup: check ref_count instead of lru before migration".
>>>
>>> Suggested-by: David Hildenbrand <david@redhat.com>
>>> Signed-off-by: Hugh Dickins <hughd@google.com>
>>> Cc: <stable@vger.kernel.org>
>>> ---
>>>    include/linux/swap.h | 10 ++++++++++
>>>    mm/gup.c             |  5 +++--
>>>    mm/mlock.c           |  6 +++---
>>>    mm/swap.c            |  2 +-
>>>    4 files changed, 17 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/include/linux/swap.h b/include/linux/swap.h
>>> index 2fe6ed2cc3fd..b49a61c32238 100644
>>> --- a/include/linux/swap.h
>>> +++ b/include/linux/swap.h
>>> @@ -385,6 +385,16 @@ void folio_add_lru_vma(struct folio *, struct
>>> vm_area_struct *);
>>>    void mark_page_accessed(struct page *);
>>>    void folio_mark_accessed(struct folio *);
>>>    
>>
>> Two smaller things:
>>
>> (1) We have other "folio_maybe_*" functions, so this one should likely
>>      better start with that as well.
>>
>> (2) With things like fscache in mind, the function can be a bit
>>      misleading.
>>
>> So I wonder if (a) we should just add kerneldoc to document it clearly (lru
>> cache, mlock cache?) and (b) maybe call it folio_may_be_lru_cached(). Not sure
>> if we can find a better abstraction for these two caches.
>>
>> Thinking again, "maybe_cached" might be a bit misleading because it implements
>> a very very very bad heuristic for small folios.
>>
>> Maybe it's more like "supports being cached".
>>
>> folio_lru_caching_supported()
> 
> folio_may_be_cached() -> folio_may_be_lru_cached(), yes, that's
> very much better, thanks.
> 
> (Settimg aside that I've never perceived those pagevecs/batches as a
> "cache"; but lru_cache_disable() gave us that terminology, and we've
> gone with the flow ever since.  lru_add_drain() would be better named
> lru_cache_drain() now, I've always got hung up on "adding a drain".)

Yeah, the terminology is not that intuitive :)

Not sure if using "batched" instead of "cached" might be clearer long-term?

> 
> "may be" rather than "maybe" was intentional: perhaps too subtle,
> but to a native speaker it neatly expresses both the "we can do this"
> and "might this have been done" cases.

I would wish we could find something that also non-native speakers can 
immediately understand ;)

"may_get_lru_cached" / "may_get_lru_batched"?

/me could not even phrase it in German properly

> 
> kernel-doc?  I don't think so, this is very much an mm-internal
> matter, and I don't care for the way kernel-doc forces us towards
> boilerplate ("@folio: The folio.") rather than helpful comment.

So a comment that this is an internal helper might be nice. Or we just 
move it straight to mm/internal.h ?

-- 
Cheers

David / dhildenb