[PATCH v1 2/2] mm/hugetlb: document why hugetlb uses folio_mapcount() for COW reuse decisions

[David Hildenbrand <david@redhat.com>]

Let's document why hugetlb still uses folio_mapcount() and is prone to
leaking memory between processes, for example using vmsplice() that
still uses FOLL_GET.

More details can be found in [1], especially around how hugetlb pages
cannot really be overcommitted, and why we don't particularly care about
these vmsplice() leaks for hugetlb -- in contrast to ordinary memory.

[1] https://lore.kernel.org/all/8b42a24d-caf0-46ef-9e15-0f88d47d2f21@redhat.com/

Suggested-by: Peter Xu <peterx@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 mm/hugetlb.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 417fc5cdb6eeb..a7efb350f5d07 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5963,6 +5963,13 @@ static vm_fault_t hugetlb_wp(struct folio *pagecache_folio,
 	/*
 	 * If no-one else is actually using this page, we're the exclusive
 	 * owner and can reuse this page.
+	 *
+	 * Note that we don't rely on the (safer) folio refcount here, because
+	 * copying the hugetlb folio when there are unexpected (temporary)
+	 * folio references could harm simple fork()+exit() users when
+	 * we run out of free hugetlb folios: we would have to kill processes
+	 * in scenarios that used to work. As a side effect, there can still
+	 * be leaks between processes, for example, with FOLL_GET users.
 	 */
 	if (folio_mapcount(old_folio) == 1 && folio_test_anon(old_folio)) {
 		if (!PageAnonExclusive(&old_folio->page)) {
-- 
2.44.0