The quilt patch titled
Subject: mm/hugetlb: document why hugetlb uses folio_mapcount() for COW reuse decisions
has been removed from the -mm tree. Its filename was
mm-hugetlb-document-why-hugetlb-uses-folio_mapcount-for-cow-reuse-decisions.patch
This patch was dropped because it was merged into the mm-stable branch
of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
------------------------------------------------------
From: David Hildenbrand <david@xxxxxxxxxx>
Subject: mm/hugetlb: document why hugetlb uses folio_mapcount() for COW reuse decisions
Date: Thu, 2 May 2024 10:52:59 +0200
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@xxxxxxxxxx/
Link: https://lkml.kernel.org/r/20240502085259.103784-3-david@xxxxxxxxxx
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
Suggested-by: Peter Xu <peterx@xxxxxxxxxx>
Cc: Muchun Song <muchun.song@xxxxxxxxx>
Cc: Shuah Khan <shuah@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/hugetlb.c | 7 +++++++
1 file changed, 7 insertions(+)
--- a/mm/hugetlb.c~mm-hugetlb-document-why-hugetlb-uses-folio_mapcount-for-cow-reuse-decisions
+++ a/mm/hugetlb.c
@@ -5963,6 +5963,13 @@ retry_avoidcopy:
/*
* 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)) {
_
Patches currently in -mm which might be from david@xxxxxxxxxx are
