linux-kernel - Re: [PATCH 1/2] mm: introduce put_user

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Date:   Thu, 13 Dec 2018 15:56:05 +0000
From:   Christopher Lameter <cl@...ux.com>
To:     Jerome Glisse <jglisse@...hat.com>
cc:     Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
        John Hubbard <jhubbard@...dia.com>,
        Matthew Wilcox <willy@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        John Hubbard <john.hubbard@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>, tom@...pey.com,
        Al Viro <viro@...iv.linux.org.uk>, benve@...co.com,
        Christoph Hellwig <hch@...radead.org>,
        "Dalessandro, Dennis" <dennis.dalessandro@...el.com>,
        Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Michal Hocko <mhocko@...nel.org>, mike.marciniszyn@...el.com,
        rcampbell@...dia.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder
 versions

On Wed, 12 Dec 2018, Jerome Glisse wrote:

> On Thu, Dec 13, 2018 at 11:51:19AM +1100, Dave Chinner wrote:
> > > > >     [O1] Avoid write back from a page still being written by either a
> > > > >          device or some direct I/O or any other existing user of GUP.
> >
> > IOWs, you need to mark pages being written to by a GUP as
> > PageWriteback, so all attempts to write the page will block on
> > wait_on_page_writeback() before trying to write the dirty page.
>
> No you don't and you can't for the simple reasons is that the GUP
> of some device driver can last days, weeks, months, years ... so
> it is not something you want to do. Here is what happens today:

I think it would be better to use the established way to block access that
Dave suggests. Maybe deal with the issue of threads being blocked for
a long time instead? Introduce a way to abort these attempts in a
controlled fashion that also allows easy debugging of these conflicts?