From: Alistair Popple <apopple@nvidia.com>
Date: Tue, 30 Aug 2022 12:01:38 +1000
Subject: mm/migrate_device.c: fix a misleading and outdated comment
Git-commit: 0742e49026121371c7ce1f640628c68c7da175d6
Patch-mainline: v6.0 or v6.0-rc8 (next release)
References: jsc#PED-1294
Commit ab09243aa95a ("mm/migrate.c: remove MIGRATE_PFN_LOCKED") changed
the way trylock_page() in migrate_vma_collect_pmd() works without updating
the comment. Reword the comment to be less misleading and a better
reflection of what happens.
Link: https://lkml.kernel.org/r/20220830020138.497063-1-apopple@nvidia.com
Fixes: ab09243aa95a ("mm/migrate.c: remove MIGRATE_PFN_LOCKED")
Signed-off-by: Alistair Popple <apopple@nvidia.com>
Reported-by: Peter Xu <peterx@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
mm/migrate_device.c | 13 ++++++++++---
1 file changed, 10 insertions(+), 3 deletions(-)
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -185,9 +185,16 @@ again:
get_page(page);
/*
- * Optimize for the common case where page is only mapped once
- * in one process. If we can lock the page, then we can safely
- * set up a special migration page table entry now.
+ * We rely on trylock_page() to avoid deadlock between
+ * concurrent migrations where each is waiting on the others
+ * page lock. If we can't immediately lock the page we fail this
+ * migration as it is only best effort anyway.
+ *
+ * If we can lock the page it's safe to set up a migration entry
+ * now. In the common case where the page is mapped once in a
+ * single process setting up the migration entry now is an
+ * optimisation to avoid walking the rmap later with
+ * try_to_migrate().
*/
if (trylock_page(page)) {
pte_t swp_pte;