The patch titled
Subject: mm/migrate_device.c: fix a misleading and outdated comment
has been added to the -mm mm-unstable branch. Its filename is
mm-migrate_devicec-fix-a-misleading-and-out-dated-comment.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-migrate_devicec-fix-a-misleading-and-out-dated-comment.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Alistair Popple <apopple@xxxxxxxxxx>
Subject: mm/migrate_device.c: fix a misleading and outdated comment
Date: Tue, 30 Aug 2022 12:01:38 +1000
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@xxxxxxxxxx
Fixes: ab09243aa95a ("mm/migrate.c: remove MIGRATE_PFN_LOCKED")
Signed-off-by: Alistair Popple <apopple@xxxxxxxxxx>
Reported-by: Peter Xu <peterx@xxxxxxxxxx>
Acked-by: Peter Xu <peterx@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/migrate_device.c | 13 ++++++++++---
1 file changed, 10 insertions(+), 3 deletions(-)
--- a/mm/migrate_device.c~mm-migrate_devicec-fix-a-misleading-and-out-dated-comment
+++ a/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)) {
bool anon_exclusive;
_
Patches currently in -mm which might be from apopple@xxxxxxxxxx are
mm-gupc-simplify-and-fix-check_and_migrate_movable_pages-return-codes.patch
selftests-hmm-tests-add-test-for-dirty-bits.patch
mm-gupc-dont-pass-gup_flags-to-check_and_migrate_movable_pages.patch
mm-gupc-refactor-check_and_migrate_movable_pages.patch
mm-migrate_devicec-fix-a-misleading-and-out-dated-comment.patch
