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

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Message-ID: <8767cde0-0ae5-4532-9460-90960bc40eef@nvidia.com>
Date: Wed, 1 May 2024 10:32:38 -0700
From: John Hubbard <jhubbard@...dia.com>
To: Jason Gunthorpe <jgg@...dia.com>, Christoph Hellwig <hch@...radead.org>
Cc: 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>,
 Alistair Popple <apopple@...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 5:10 AM, Jason Gunthorpe wrote:
> 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.

So do I. :)

> 
> +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..
> 

OK, I'll work on finding out what is temporarily elevating the refcount
and preventing the migration. And see where that leads.


thanks,
-- 
John Hubbard
NVIDIA