linux-kernel - [PATCH v1] mm/gup: Revert "mm: gup: fix infinite loop within __get_longterm

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Message-ID: <20250611131314.594529-1-david@redhat.com>
Date: Wed, 11 Jun 2025 15:13:14 +0200
From: David Hildenbrand <david@...hat.com>
To: linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org,
	David Hildenbrand <david@...hat.com>,
	Hyesoo Yu <hyesoo.yu@...sung.com>,
	Stable@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jason Gunthorpe <jgg@...pe.ca>,
	Peter Xu <peterx@...hat.com>,
	Zhaoyang Huang <zhaoyang.huang@...soc.com>,
	Aijun Sun <aijun.sun@...soc.com>,
	Alistair Popple <apopple@...dia.com>,
	John Hubbard <jhubbard@...dia.com>
Subject: [PATCH v1] mm/gup: Revert "mm: gup: fix infinite loop within __get_longterm_locked"

After commit 1aaf8c122918 ("mm: gup: fix infinite loop within
__get_longterm_locked") we are able to longterm pin folios that are not
supposed to get longterm pinned, simply because they temporarily have
the LRU flag cleared (esp. temporarily isolated).

For example, two __get_longterm_locked() callers can race, or
__get_longterm_locked() can race with anything else that temporarily
isolates folios.

The introducing commit mentions the use case of a driver that uses
vm_ops->fault to insert pages allocated through cma_alloc() into the
page tables, assuming they can later get longterm pinned. These pages/
folios would never have the LRU flag set and consequently cannot get
isolated. There is no known in-tree user making use of that so far,
fortunately.

To handle that in the future -- and avoid retrying forever to
isolate/migrate them -- we will need a different mechanism for the CMA
area *owner* to indicate that it actually already allocated the page and
is fine with longterm pinning it. The LRU flag is not suitable for that.

Probably we can lookup the relevant CMA area and query the bitmap; we
only have have to care about some races, probably. If already allocated,
we could just allow longterm pinning)

Anyhow, let's fix the "must not be longterm pinned" problem first by
reverting the original commit.

Fixes: 1aaf8c122918 ("mm: gup: fix infinite loop within __get_longterm_locked")
Closes: https://lore.kernel.org/all/20250522092755.GA3277597@tiffany/
Reported-by: Hyesoo Yu <hyesoo.yu@...sung.com>
Cc: <Stable@...r.kernel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Jason Gunthorpe <jgg@...pe.ca>
Cc: Peter Xu <peterx@...hat.com>
Cc: Zhaoyang Huang <zhaoyang.huang@...soc.com>
Cc: Aijun Sun <aijun.sun@...soc.com>
Cc: Alistair Popple <apopple@...dia.com>
Cc: John Hubbard <jhubbard@...dia.com>
Signed-off-by: David Hildenbrand <david@...hat.com>
---
 mm/gup.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index e065a49842a87..3c39cbbeebef1 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2303,13 +2303,13 @@ static void pofs_unpin(struct pages_or_folios *pofs)
 /*
  * Returns the number of collected folios. Return value is always >= 0.
  */
-static void collect_longterm_unpinnable_folios(
+static unsigned long collect_longterm_unpinnable_folios(
 		struct list_head *movable_folio_list,
 		struct pages_or_folios *pofs)
 {
+	unsigned long i, collected = 0;
 	struct folio *prev_folio = NULL;
 	bool drain_allow = true;
-	unsigned long i;

 	for (i = 0; i < pofs->nr_entries; i++) {
 		struct folio *folio = pofs_get_folio(pofs, i);
@@ -2321,6 +2321,8 @@ static void collect_longterm_unpinnable_folios(
 		if (folio_is_longterm_pinnable(folio))
 			continue;

+		collected++;
+
 		if (folio_is_device_coherent(folio))
 			continue;

@@ -2342,6 +2344,8 @@ static void collect_longterm_unpinnable_folios(
 				    NR_ISOLATED_ANON + folio_is_file_lru(folio),
 				    folio_nr_pages(folio));
 	}
+
+	return collected;
 }

 /*
@@ -2418,9 +2422,11 @@ static long
 check_and_migrate_movable_pages_or_folios(struct pages_or_folios *pofs)
 {
 	LIST_HEAD(movable_folio_list);
+	unsigned long collected;

-	collect_longterm_unpinnable_folios(&movable_folio_list, pofs);
-	if (list_empty(&movable_folio_list))
+	collected = collect_longterm_unpinnable_folios(&movable_folio_list,
+						       pofs);
+	if (!collected)
 		return 0;

 	return migrate_longterm_unpinnable_folios(&movable_folio_list, pofs);
-- 
2.49.0