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Date: Sun, 17 Feb 2019 09:42:50 +1100 From: Dave Chinner <david@...morbit.com> To: Ira Weiny <ira.weiny@...el.com> Cc: Jason Gunthorpe <jgg@...pe.ca>, Christopher Lameter <cl@...ux.com>, Matthew Wilcox <willy@...radead.org>, Jerome Glisse <jglisse@...hat.com>, Dan Williams <dan.j.williams@...el.com>, Jan Kara <jack@...e.cz>, Doug Ledford <dledford@...hat.com>, lsf-pc@...ts.linux-foundation.org, linux-rdma <linux-rdma@...r.kernel.org>, Linux MM <linux-mm@...ck.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, John Hubbard <jhubbard@...dia.com>, Michal Hocko <mhocko@...nel.org> Subject: Re: [LSF/MM TOPIC] Discuss least bad options for resolving longterm-GUP usage by RDMA 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@...morbit.com
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