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

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Date: Wed, 1 May 2024 23:49:48 -0700
From: John Hubbard <jhubbard@...dia.com>
To: Alistair Popple <apopple@...dia.com>, Jason Gunthorpe <jgg@...dia.com>
Cc: Christoph Hellwig <hch@...radead.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 LKML <linux-kernel@...r.kernel.org>, linux-rdma@...r.kernel.org,
 linux-mm@...ck.org, Mike Marciniszyn <mike.marciniszyn@...el.com>,
 Leon Romanovsky <leon@...nel.org>, Artemy Kovalyov <artemyko@...dia.com>,
 Michael Guralnik <michaelgur@...dia.com>, Pak Markthub <pmarkthub@...dia.com>
Subject: Re: [RFC] RDMA/umem: pin_user_pages*() can temporarily fail due to
 migration glitches

On 5/1/24 6:05 PM, Alistair Popple wrote:
> Jason Gunthorpe <jgg@...dia.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