mm/gup: factor out LRU cache draining for folio into lru_cache_drain_for_folio()

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.

Link: https://lore.kernel.org/20260806-lru_cache_drain_for_folio-v1-1-c6287d295e99@kernel.org
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Fuad Tabba <fuad.tabba@linux.dev>
Cc: Ackerley Tng <ackerleytng@google.com>
Cc: Baoquan He <baoquan.he@linux.dev>
Cc: Barry Song <baohua@kernel.org>
Cc: Chris Li <chrisl@kernel.org>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Kairui Song <kasong@tencent.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 8 insertions, 0 deletions

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 330a420fd6de..b4b1c0a84c8b 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -298,6 +298,14 @@ void folio_add_lru(struct folio *folio);
 void folio_mark_accessed(struct folio *folio);
 void lru_add_drain_all(void);
 
+enum lru_cache_drained {
+	LRU_CACHE_NOT_DRAINED,
+	LRU_CACHE_DRAINED,
+	LRU_CACHE_DRAINED_ALL,
+};
+void lru_cache_drain_for_folio(const struct folio *folio,
+		unsigned int extra_refs, enum lru_cache_drained *drained);
+
 /* linux/mm/folio-compat.c */
 void mark_page_accessed(struct page *page);