From 0742e49026121371c7ce1f640628c68c7da175d6 Mon Sep 17 00:00:00 2001 From: Alistair Popple Date: Tue, 30 Aug 2022 12:01:38 +1000 Subject: mm/migrate_device.c: fix a misleading and outdated comment Commit ab09243aa95a ("mm/migrate.c: remove MIGRATE_PFN_LOCKED") changed the way trylock_page() in migrate_vma_collect_pmd() works without updating the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 Reported-by: Peter Xu Acked-by: Peter Xu Signed-off-by: Andrew Morton --- mm/migrate_device.c | 13 ++++++++++--- 1 file changed, 10 insertions(+), 3 deletions(-) (limited to 'mm/migrate_device.c') diff --git a/mm/migrate_device.c b/mm/migrate_device.c index 27fb37d65476..72354248dde9 100644 --- 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)) { bool anon_exclusive; -- cgit v1.2.3