linux-kernel - Re: [LSF/MM TOPIC] Discuss least bad options for resolving longterm-GUP usage by RDMA

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Date:   Mon, 11 Feb 2019 14:33:53 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
CC:     Ira Weiny <ira.weiny@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Jan Kara <jack@...e.cz>, Dave Chinner <david@...morbit.com>,
        Christopher Lameter <cl@...ux.com>,
        Doug Ledford <dledford@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        <lsf-pc@...ts.linux-foundation.org>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jerome Glisse <jglisse@...hat.com>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [LSF/MM TOPIC] Discuss least bad options for resolving
 longterm-GUP usage by RDMA

On 2/11/19 2:12 PM, Jason Gunthorpe wrote:
> On Mon, Feb 11, 2019 at 01:22:11PM -0800, John Hubbard wrote:
> 
>> The only way that breaks is if longterm pins imply an irreversible action, such
>> as blocking and waiting in a way that you can't back out of or get interrupted
>> out of. And the design doesn't seem to be going in that direction, right?
> 
> RDMA, vfio, etc will always have 'long term' pins that are
> irreversible on demand. It is part of the HW capability.
> 

Yes, I get that about the HW. But I didn't quite phrase it accurately. What I
meant was, irreversible from the kernel code's point of view; specifically,
the filesystem while in various writeback paths.

But anyway, Jan's proposal a bit earlier today [1] is finally sinking into
my head--if we actually go that way, and prevent the caller from setting up
a problematic gup pin in the first place, then that may make this point sort
of moot.


> I think the flag is badly named, it is really more of a
> GUP_LOCK_PHYSICAL_ADDRESSES flag.
> 
> ie indicate to the FS that is should not attempt to remap physical
> memory addresses backing this VMA. If the FS can't do that it must
> fail.
> 

Yes. Duration is probably less important than the fact that the page
is specially treated.

[1] https://lore.kernel.org/r/20190211102402.GF19029@quack2.suse.cz
thanks,
-- 
John Hubbard
NVIDIA