mm: gup: fix infinite loop within __get_longterm_locked

commit 1aaf8c1
Author: Zhaoyang Huang <[email protected]>
Date:   Tue Jan 21 10:01:59 2025 +0800

    mm: gup: fix infinite loop within __get_longterm_locked

    We can run into an infinite loop in __get_longterm_locked() when
    collect_longterm_unpinnable_folios() finds only folios that are isolated
    from the LRU or were never added to the LRU.  This can happen when all
    folios to be pinned are never added to the LRU, for example when
    vm_ops->fault allocated pages using cma_alloc() and never added them to
    the LRU.

    Fix it by simply taking a look at the list in the single caller, to see if
    anything was added.

    [[email protected]: move definition of local]
      Link: https://lkml.kernel.org/r/[email protected]
    Link: https://lkml.kernel.org/r/[email protected]
    Fixes: 67e139b ("mm/gup.c: refactor check_and_migrate_movable_pages()")
    Signed-off-by: Zhaoyang Huang <[email protected]>
    Reviewed-by: John Hubbard <[email protected]>
    Reviewed-by: David Hildenbrand <[email protected]>
    Suggested-by: David Hildenbrand <[email protected]>
    Acked-by: David Hildenbrand <[email protected]>
    Cc: Aijun Sun <[email protected]>
    Cc: Alistair Popple <[email protected]>
    Cc: <[email protected]>
    Signed-off-by: Andrew Morton <[email protected]>

JIRA: https://issues.redhat.com/browse/RHEL-77742
Signed-off-by: Nico Pache <[email protected]>

Author: PacheNico

mm/gup.c

@@ -2323,13 +2323,13 @@ static void pofs_unpin(struct pages_or_folios *pofs)
 /*
  * Returns the number of collected folios. Return value is always >= 0.
  */
-static unsigned long collect_longterm_unpinnable_folios(
+static void 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);
@@ -2341,8 +2341,6 @@ static unsigned long collect_longterm_unpinnable_folios(
 		if (folio_is_longterm_pinnable(folio))
 			continue;
 
-		collected++;
-
 		if (folio_is_device_coherent(folio))
 			continue;
 
@@ -2364,8 +2362,6 @@ static unsigned long collect_longterm_unpinnable_folios(
 				    NR_ISOLATED_ANON + folio_is_file_lru(folio),
 				    folio_nr_pages(folio));
 	}
-
-	return collected;
 }
 
 /*
@@ -2442,11 +2438,9 @@ static long
 check_and_migrate_movable_pages_or_folios(struct pages_or_folios *pofs)
 {
 	LIST_HEAD(movable_folio_list);
-	unsigned long collected;
 
-	collected = collect_longterm_unpinnable_folios(&movable_folio_list,
-						       pofs);
-	if (!collected)
+	collect_longterm_unpinnable_folios(&movable_folio_list, pofs);
+	if (list_empty(&movable_folio_list))
 		return 0;
 
 	return migrate_longterm_unpinnable_folios(&movable_folio_list, pofs);