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

On 8/7/26 23:52, Ackerley Tng via B4 Relay wrote:

From: "David Hildenbrand (Arm)" <david@kernel.org>

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@kernel.org>
Signed-off-by: Ackerley Tng <redacted>
---

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

-- 
Cheers,

David