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 da...@fromorbit.com
