On 12.08.24 07:35, Dev Jain wrote:
On 8/11/24 14:38, David Hildenbrand wrote:
On 11.08.24 08:06, Dev Jain wrote:
On 8/11/24 00:22, David Hildenbrand wrote:
On 10.08.24 20:42, Dev Jain wrote:
On 8/9/24 19:17, David Hildenbrand wrote:
On 09.08.24 12:31, Dev Jain wrote:
As already being done in __migrate_folio(), wherein we backoff if
the
folio refcount is wrong, make this check during the unmapping phase,
upon
the failure of which, the original state of the PTEs will be
restored
and
the folio lock will be dropped via migrate_folio_undo_src(), any
racing
thread will make progress and migration will be retried.
Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
---
mm/migrate.c | 9 +++++++++
1 file changed, 9 insertions(+)
diff --git a/mm/migrate.c b/mm/migrate.c
index e7296c0fb5d5..477acf996951 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1250,6 +1250,15 @@ static int migrate_folio_unmap(new_folio_t
get_new_folio,
}
if (!folio_mapped(src)) {
+ /*
+ * Someone may have changed the refcount and maybe sleeping
+ * on the folio lock. In case of refcount mismatch, bail
out,
+ * let the system make progress and retry.
+ */
+ struct address_space *mapping = folio_mapping(src);
+
+ if (folio_ref_count(src) != folio_expected_refs(mapping,
src))
+ goto out;
This really seems to be the latest point where we can "easily" back
off and unlock the source folio -- in this function :)
I wonder if we should be smarter in the migrate_pages_batch() loop
when we start the actual migrations via migrate_folio_move(): if we
detect that a folio has unexpected references *and* it has waiters
(PG_waiters), back off then and retry the folio later. If it only has
unexpected references, just keep retrying: no waiters -> nobody is
waiting for the lock to make progress.
The patch currently retries migration irrespective of the reason of
refcount change.
If you are suggesting that, break the retrying according to two
conditions:
That's not what I am suggesting ...
This really seems to be the latest point where we can "easily" back
off and unlock the source folio -- in this function :)
For example, when migrate_folio_move() fails with -EAGAIN, check if
there are waiters (PG_waiter?) and undo+unlock to try again later.
Currently, on -EAGAIN, migrate_folio_move() returns without undoing
src
and dst; even if we were to fall
...
I am wondering if we should detect here if there are waiters and undo
src+dst.
After undoing src+dst, which restores the PTEs, how are you going to
set the
PTEs to migration again? That is being done through
migrate_folio_unmap(),
and the loops of _unmap() and _move() are different. Or am I missing
something...
Again, no expert on the code, but it would mean that if we detect that
there are waiters, we would undo src+dst and add them to ret_folios,
similar to what we do in "Cleanup remaining folios" at the end of
migrate_pages_batch()?
So instead of retrying migration of that folio, just give it up
immediately and retry again later.
Of course, this means that (without further modifications to that
function), we would leave retrying these folios to the caller, such as
in migrate_pages_sync(), where we move ret_folios to the tail of
"folios" and retry migration.
So IIUC, you are saying to change the return value in
__folio_migrate_mapping(), so that when move_to_new_folio() fails
in migrate_folio_move(), we end up in the retrying loop of _sync() which
calls _batch() in synchronous mode. Here, we
will have to make a change to decide how much we want to retry?
So essentially, instead of checking for "unexpected references" and
backing off once at the beginning (what you do in this patch), we would
*not* add new checks for "unexpected references" and not fail early in
that case.
Instead, we would continuously check if there are waiters, and if there
are waiters, we back-off completely (->unlock) instead of retrying
something that cannot possibly make progress.
For "unexpected references" it can make sense to just retry immediately,
because these might just be speculative references or short-term
references that will go away soon.
For "unexpected reference with waiters" (or just "waiters" which should
be the same because "waiters" should imply "unexpected references"),
it's different as you discovered.
What we do with these "somebody else is waiting to make progress" pages
is indeed a god question -- Ying seems to have some ideas in how to
optimize retrying further.
--
Cheers,
David / dhildenb
