linux-kernel - Re: [RFC 08/16] vfio/type1: Cache locked_vm to ease mmap

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Date:   Fri, 7 Jan 2022 11:18:07 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Daniel Jordan <daniel.m.jordan@...cle.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        Alexander Duyck <alexanderduyck@...com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ben Segall <bsegall@...gle.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Ingo Molnar <mingo@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Josh Triplett <josh@...htriplett.org>,
        Michal Hocko <mhocko@...e.com>, Nico Pache <npache@...hat.com>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        Steve Sistare <steven.sistare@...cle.com>,
        Tejun Heo <tj@...nel.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-mm@...ck.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org
Subject: Re: [RFC 08/16] vfio/type1: Cache locked_vm to ease mmap_lock
 contention

On Thu, Jan 06, 2022 at 10:06:42PM -0500, Daniel Jordan wrote:

> > At least it seems like it is not an insurmountable problem if it makes
> > an appreciable difference..
> 
> Ok, I can think more about this.

Unfortunately iommufd is not quite ready yet, otherwise I might
suggest just focus on that not type 1 optimizations. Depends on your
timeframe I suppose.

> > After seeing Daniels's patches I've been wondering if the pin step in
> > iommufd's draft could be parallized on a per-map basis without too
> > much trouble. It might give Daniel a way to do a quick approach
> > comparison..
> 
> Sorry, comparison between what?  I can take a look at iommufd tomorrow
> though and see if your comment makes more sense.

I think it might be easier to change the iommufd locking than the
type1 locking to allow kernel-side parallel map ioctls. It is already
almost properly locked for this right now, just the iopt lock covers a
little bit too much.

It could give some idea what kind of performance user managed
concurrency gives vs kernel auto threading.

Jason