| From: Alistair Popple <apopple@nvidia.com> |
| Subject: mm/migrate_device.c: fix a misleading and outdated comment |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@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> |
| --- |
| |
| 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; |
| _ |
