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

From: Christopher Lameter
Date: Wed Feb 06 2019 - 15:24:21 EST

On Wed, 6 Feb 2019, Matthew Wilcox wrote:

> >
> > 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".

Hmmm... I thought that a file handle must have a special open mode to
actually to a dax map. Looks like that is not the case.

> > 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.

Well but that requires that the application unmaps the file.

> > 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.

DAX provides a mapping to physical persistent memory that
does not go away. Or its a block device.

> > 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?

Well that is controlled by mlock and could also involve other means like
disabling swap.