Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions

[Christopher Lameter]

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?