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

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Message-ID: <a2032a79-744d-4c00-a286-7d6fed3a1bdb@nvidia.com>
Date: Thu, 2 May 2024 11:10:05 -0700
From: John Hubbard <jhubbard@...dia.com>
To: David Hildenbrand <david@...hat.com>, 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 11:56 PM, David Hildenbrand wrote:
> On 02.05.24 03:05, Alistair Popple wrote:
>> Jason Gunthorpe <jgg@...dia.com> writes:
..
>>>> 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?
> 
> There are some weird things that still needs fixing: vmsplice() is the 
> example that doesn't use FOLL_LONGTERM.
> 

Hi David!

Do you have any other call sites in mind? It sounds like one way forward
is to fix each call site...

This is an unhappy story right now: the pin_user_pages*() APIs are
significantly worse than before the "migrate pages away automatically"
upgrade, from a user point of view. Because now, the APIs fail
intermittently for callers who follow the rules--because
pin_user_pages() is not fully working yet, basically.

Other ideas, large or small, about how to approach a fix?

thanks,

-- 
John Hubbard
NVIDIA