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

From: Christopher Lameter
Date: Fri Feb 15 2019 - 10:42:06 EST

On Fri, 15 Feb 2019, Dave Chinner wrote:

> Which tells us filesystem people that the applications are doing
> something that _will_ cause data corruption and hence not to spend
> any time triaging data corruption reports because it's not a
> filesystem bug that caused it.
> See open(2):
> Applications should avoid mixing O_DIRECT and normal I/O to
> the same file, and especially to overlapping byte regions in
> the same file. Even when the filesystem correctly handles
> the coherency issues in this situation, overall I/O
> throughput is likely to be slower than using either mode
> alone. Likewise, applications should avoid mixing mmap(2)
> of files with direct I/O to the same files.

Since RDMA is something similar: Can we say that a file that is used for
RDMA should not use the page cache?

And can we enforce this in the future? I.e. have some file state that says
that this file is direct/RDMA or contains long term pinning and thus
allows only a certain type of operations to ensure data consistency?

If we cannot enforce it then we may want to spit out some warning?