linux-kernel - Re: [PATCH v8 5/8] mm: Device exclusive memory access

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Message-ID: <2569629.VzlulnA7BY@nvdebian>
Date:   Wed, 19 May 2021 23:11:55 +1000
From:   Alistair Popple <apopple@...dia.com>
To:     Peter Xu <peterx@...hat.com>
CC:     Jason Gunthorpe <jgg@...dia.com>, <linux-mm@...ck.org>,
        <nouveau@...ts.freedesktop.org>, <bskeggs@...hat.com>,
        <akpm@...ux-foundation.org>, <linux-doc@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <dri-devel@...ts.freedesktop.org>,
        <jhubbard@...dia.com>, <rcampbell@...dia.com>,
        <jglisse@...hat.com>, <hch@...radead.org>, <daniel@...ll.ch>,
        <willy@...radead.org>, <bsingharora@...il.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v8 5/8] mm: Device exclusive memory access

On Wednesday, 19 May 2021 10:15:41 PM AEST Peter Xu wrote:
> External email: Use caution opening links or attachments
> 
> On Wed, May 19, 2021 at 09:04:53PM +1000, Alistair Popple wrote:
> > Failing fork() because we couldn't take a lock doesn't seem like the right
> > approach though, especially as there is already existing code that
> > retries. I get this adds complexity though, so would be happy to take a
> > look at cleaning copy_pte_range() up in future.
> 
> Yes, I proposed that as this one won't affect any existing applications
> (unlike the existing ones) but only new userspace driver apps that will use
> this new atomic feature.
> 
> IMHO it'll be a pity to add extra complexity and maintainance burden into
> fork() if only for keeping the "logical correctness of fork()" however the
> code never triggers. If we start with trylock we'll know whether people
> will use it, since people will complain with a reason when needed; however
> I still doubt whether a sane userspace device driver should fork() within
> busy interaction with the device underneath..

I will refrain from commenting on the sanity or otherwise of doing that :-)

Agree such a scenario seems unlikely in practice (and possibly unreasonable). 
Keeping the "logical correctness of fork()" still seems worthwhile to me, but 
if the added complexity/maintenance burden for an admittedly fairly specific 
feature is going to stop progress here I am happy to take the fail fork 
approach. I could then possibly fix it up as a future clean up to 
copy_pte_range(). Perhaps others have thoughts?

> In all cases, please still consider to keep them in copy_nonpresent_pte()
> (and if to rework, separating patches would be great).
>
> Thanks,
> 
> --
> Peter Xu