Re: [RFC] RDMA/umem: pin_user_pages*() can temporarily fail due to migration glitches

John Hubbard
2024-05-02T06:50:02Z

On 5/1/24 6:05 PM, Alistair Popple wrote:
> Jason Gunthorpe <jgg@nvidia.com> writes:
>> On Tue, Apr 30, 2024 at 10:10:43PM -0700, Christoph Hellwig wrote:
...
>>> This doesn't make sense. IFF a blind retry is all that is needed it
>>> should be done in the core functionality. I fear it's not that easy,
>>> though.
>>
>> +1
>>
>> This migration retry weirdness is a GUP issue, it needs to be solved
>> in the mm not exposed to every pin_user_pages caller.
>>
>> If it turns out ZONE_MOVEABLE pages can't actually be reliably moved
>> then it is pretty broken..
>
> I wonder if we should remove the arbitrary retry limit in
> migrate_pages() entirely for ZONE_MOVEABLE pages and just loop until
> they migrate? By definition there should only be transient references on
> these pages so why do we need to limit the number of retries in the
> first place?
>

Well, along those lines, I can confirm that this patch also fixes the
symptoms:

diff --git a/mm/migrate.c b/mm/migrate.c
index 73a052a382f1..faa67cc441e2 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1728,7 +1728,9 @@ static int migrate_pages_batch(struct list_head *from,
else
goto move;
case -EAGAIN:
- retry++;
+ /* For ZONE_MOVABLE folios, retry forever */
+ if (!folio_is_zone_movable(folio))
+ retry++;
thp_retry += is_thp;
nr_retry_pages += nr_pages;
break;
@@ -1786,7 +1788,9 @@ static int migrate_pages_batch(struct list_head *from,
*/
switch(rc) {
case -EAGAIN:
- retry++;
+ /* For ZONE_MOVABLE folios, retry forever */
+ if (!folio_is_zone_movable(folio))
+ retry++;
thp_retry += is_thp;
nr_retry_pages += nr_pages;
break;

thanks,
--
John Hubbard
NVIDIA