Re: [PATCH v10 14/41] mm/gup: factor out LRU cache draining for folio into lru_cache_drain_for_folio()

[David Hildenbrand (Arm)]

On 8/7/26 23:52, Ackerley Tng via B4 Relay wrote:
> From: "David Hildenbrand (Arm)" <david@xxxxxxxxxx>
> 
> KVM with guest_memfd wants to remove any folio references due to LRU
> caches, as it really must only allow to convert folios from shared to
> private when there are no unexpected folio references (e.g., from GUP
> references).
> 
> So, to drive the refcount down, it needs a way to flush the LRU caches.
> Let's factor out what we have in lru_cache_drain_for_folio(). Document
> it, and also mention that concurrent folio (un)mapping might, in theory,
> miss detecting LRU cache references. Keep obtaining the expected refcount
> twice to minimize the possibility. For the current and future user that
> should work, and we don't really have a better alternative: we could
> detect if the mapcount changed, but it would still be racy and add more
> complexity with questionable benefit.
> 
> Maybe there is a chance to avoid the draining entirely in the future,
> by avoiding extra references from the LRU cache: Hugh thinks there might
> be a way. But for the time being, this handling is unfortunately
> required.
> 
> Make folio_may_be_lru_cached() accept a const pointer so
> lru_cache_drain_for_folio() can accept a const pointer as well.
> 
> Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
> ---

If all goes well, this patch should go into 7.3-rc1. So once you rebase, you'd
get it for free :)

-- 
Cheers,

David