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

[Dave Chinner]

On Fri, Feb 15, 2019 at 03:38:29PM -0800, Ira Weiny wrote:
> On Fri, Feb 15, 2019 at 03:00:31PM -0700, Jason Gunthorpe wrote:
> > On Fri, Feb 15, 2019 at 06:31:36PM +0000, Christopher Lameter wrote:
> > > On Fri, 15 Feb 2019, Matthew Wilcox wrote:
> > > 
> > > > > Since RDMA is something similar: Can we say that a file that is used for
> > > > > RDMA should not use the page cache?
> > > >
> > > > That makes no sense.  The page cache is the standard synchronisation point
> > > > for filesystems and processes.  The only problems come in for the things
> > > > which bypass the page cache like O_DIRECT and DAX.
> > > 
> > > It makes a lot of sense since the filesystems play COW etc games with the
> > > pages and RDMA is very much like O_DIRECT in that the pages are modified
> > > directly under I/O. It also bypasses the page cache in case you have
> > > not noticed yet.
> > 
> > It is quite different, O_DIRECT modifies the physical blocks on the
> > storage, bypassing the memory copy.
> >
> 
> Really?  I thought O_DIRECT allowed the block drivers to write to/from user
> space buffers.  But the _storage_ was still under the control of the block
> drivers?

Yup, in a nutshell. Even O_DIRECT on DAX doesn't modify the physical
storage directly - it ends up in the pmem driver and it does a
memcpy() to move the data to/from the physical storage and the user
space buffer. It's exactly the same IO path as moving data to/from
the physical storage into the page cache pages....

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx