[merged mm-stable] mm-hugetlb-document-why-hugetlb-uses-folio_mapcount-for-cow-reuse-decisions.patch removed from -mm tree

[merged mm-stable] mm-hugetlb-document-why-hugetlb-uses-folio_mapcount-for-cow-reuse-decisions.patch removed from -mm tree

[Andrew Morton]

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@redhat.com>
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@redhat.com/

Link: https://lkml.kernel.org/r/20240502085259.103784-3-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Suggested-by: Peter Xu <peterx@redhat.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 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@redhat.com are