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

[Matthew Wilcox <willy@infradead.org>]

On Wed, Feb 06, 2019 at 07:16:21PM +0000, Christopher Lameter wrote:
> On Wed, 6 Feb 2019, Doug Ledford wrote:
> > > Most of the cases we want revoke for are things like truncate().
> > > Shouldn't happen with a sane system, but we're trying to avoid users
> > > doing awful things like being able to DMA to pages that are now part of
> > > a different file.
> >
> > Why is the solution revoke then?  Is there something besides truncate
> > that we have to worry about?  I ask because EBUSY is not currently
> > listed as a return value of truncate, so extending the API to include
> > EBUSY to mean "this file has pinned pages that can not be freed" is not
> > (or should not be) totally out of the question.
> >
> > Admittedly, I'm coming in late to this conversation, but did I miss the
> > portion where that alternative was ruled out?
> 
> Coming in late here too but isnt the only DAX case that we are concerned
> about where there was an mmap with the O_DAX option to do direct write

There is no O_DAX option.  There's mount -o dax, but there's nothing that
a program does to say "Use DAX".

> though? If we only allow this use case then we may not have to worry about
> long term GUP because DAX mapped files will stay in the physical location
> regardless.

... except for truncate.  And now that I think about it, there was a
desire to support hot-unplug which also needed revoke.

> Maybe we can solve the long term GUP problem through the requirement that
> user space acquires some sort of means to pin the pages? In the DAX case
> this is given by the filesystem and the hardware will basically take care
> of writeback.

It's not given by the filesystem.

> In case of anonymous memory this can be guaranteed otherwise and is less
> critical since these pages are not part of the pagecache and are not
> subject to writeback.

but are subject to being swapped out?