* [PATCH v2] mm/migrate_device.c: Fix a misleading and out-dated comment
@ 2022-08-30 2:01 Alistair Popple
2022-08-30 15:32 ` Peter Xu
0 siblings, 1 reply; 2+ messages in thread
From: Alistair Popple @ 2022-08-30 2:01 UTC (permalink / raw)
To: Andrew Morton
Cc: David Hildenbrand, linux-mm, John Hubbard, Ralph Campbell,
Alistair Popple, Peter Xu
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.
Signed-off-by: Alistair Popple <apopple@nvidia.com>
Reported-by: Peter Xu <peterx@redhat.com>
Fixes: ab09243aa95a ("mm/migrate.c: remove MIGRATE_PFN_LOCKED")
---
mm/migrate_device.c | 13 ++++++++++---
1 file changed, 10 insertions(+), 3 deletions(-)
diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index 5052093d0262..9e7ec925ec51 100644
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -179,9 +179,16 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
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;
--
2.35.1
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH v2] mm/migrate_device.c: Fix a misleading and out-dated comment
2022-08-30 2:01 [PATCH v2] mm/migrate_device.c: Fix a misleading and out-dated comment Alistair Popple
@ 2022-08-30 15:32 ` Peter Xu
0 siblings, 0 replies; 2+ messages in thread
From: Peter Xu @ 2022-08-30 15:32 UTC (permalink / raw)
To: Alistair Popple
Cc: Andrew Morton, David Hildenbrand, linux-mm, John Hubbard, Ralph Campbell
On Tue, Aug 30, 2022 at 12:01:38PM +1000, Alistair Popple wrote:
> 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.
>
> Signed-off-by: Alistair Popple <apopple@nvidia.com>
> Reported-by: Peter Xu <peterx@redhat.com>
> Fixes: ab09243aa95a ("mm/migrate.c: remove MIGRATE_PFN_LOCKED")
Acked-by: Peter Xu <peterx@redhat.com>
Thanks,
--
Peter Xu
^ permalink raw reply [flat|nested] 2+ messages in thread
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2022-08-30 2:01 [PATCH v2] mm/migrate_device.c: Fix a misleading and out-dated comment Alistair Popple
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