[PATCH v3 1/1] mm: gup: avoid CMA page pinning by retrying migration if no migratable page

[Hyesoo Yu <hyesoo.yu@samsung.com>]

Commit 1aaf8c122918 ("mm: gup: fix infinite loop within __get_longterm_locked")
introduced an issue where CMA pages could be pinned by longterm GUP requests.
This occurs when unpinnable pages are detected but the movable_page_list is empty;
the commit would return success without retrying, allowing unpinnable
pages (such as CMA) to become pinned.

CMA pages may be temporarily off the LRU due to concurrent isolation,
for example when multiple longterm GUP requests are racing and therefore
not appear in movable_page_list. Before commit 1aaf8c, the kernel would
retry migration in such cases, which helped avoid accidental CMA pinning.

The original intent of the commit was to support longterm GUP on non-LRU
CMA pages in out-of-tree use cases such as pKVM. However, allowing this
can lead to broader CMA pinning issues.

To avoid this, the logic is restored to return -EAGAIN instead of success
when no folios could be collected but unpinnable pages were found.
This ensures that migration is retried until success, and avoids
inadvertently pinning unpinnable pages.

Fixes: 1aaf8c122918 ("mm: gup: fix infinite loop within __get_longterm_locked")
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Hyesoo Yu <hyesoo.yu@samsung.com>

---
We have confirmed that this regression causes CMA pages to be pinned
in our kernel 6.12-based environment.

In addition to CMA allocation failures, we also observed longterm GUP failures
when repeatedly accessing the same VMA. Specifically, the first longterm GUP
call would pin a CMA page, and a second call on the same region
would fail the migration because the cma page was already pinned.

After reverting commit 1aaf8c122918, the issue no longer reproduced.

Therefore, this fix is important to ensure reliable behavior of longterm GUP
and CMA-backed memory, and should be backported to stable.
---
 mm/gup.c | 28 ++++++++++++++++++++++------
 1 file changed, 22 insertions(+), 6 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index e065a49842a8..66193421c1d4 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2300,14 +2300,12 @@ static void pofs_unpin(struct pages_or_folios *pofs)
 		unpin_user_pages(pofs->pages, pofs->nr_entries);
 }
 
-/*
- * Returns the number of collected folios. Return value is always >= 0.
- */
-static void collect_longterm_unpinnable_folios(
+static bool collect_longterm_unpinnable_folios(
 		struct list_head *movable_folio_list,
 		struct pages_or_folios *pofs)
 {
 	struct folio *prev_folio = NULL;
+	bool any_unpinnable = false;
 	bool drain_allow = true;
 	unsigned long i;
 
@@ -2321,6 +2319,8 @@ static void collect_longterm_unpinnable_folios(
 		if (folio_is_longterm_pinnable(folio))
 			continue;
 
+		any_unpinnable = true;
+
 		if (folio_is_device_coherent(folio))
 			continue;
 
@@ -2342,6 +2342,8 @@ static void collect_longterm_unpinnable_folios(
 				    NR_ISOLATED_ANON + folio_is_file_lru(folio),
 				    folio_nr_pages(folio));
 	}
+
+	return any_unpinnable;
 }
 
 /*
@@ -2417,11 +2419,25 @@ migrate_longterm_unpinnable_folios(struct list_head *movable_folio_list,
 static long
 check_and_migrate_movable_pages_or_folios(struct pages_or_folios *pofs)
 {
+	bool any_unpinnable;
+
 	LIST_HEAD(movable_folio_list);
 
-	collect_longterm_unpinnable_folios(&movable_folio_list, pofs);
-	if (list_empty(&movable_folio_list))
+	any_unpinnable = collect_longterm_unpinnable_folios(&movable_folio_list, pofs);
+
+	if (list_empty(&movable_folio_list)) {
+		/*
+		 * If we find any longterm unpinnable page that we failed to
+		 * isolated for migration, it might be because someone else
+		 * concurrently isolated it. Make the caller retry until it
+		 * succeeds.
+		 */
+		if (any_unpinnable) {
+			pofs_unpin(pofs);
+			return -EAGAIN;
+		}
 		return 0;
+	}
 
 	return migrate_longterm_unpinnable_folios(&movable_folio_list, pofs);
 }
-- 
2.49.0