Re: [Qemu-devel] [Xen-devel] [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes.

[Alex Bligh <alex@alex.org.uk>]

--On 19 March 2013 12:14:36 +0100 Paolo Bonzini <pbonzini@redhat.com> wrote:

> Unfortunately no.  The problem is in the loading side's kernel, on which
> you do not have any control.  If the loading side doesn't use O_DIRECT,
> any attempt to invalidate the metadata in userspace or on the source is
> futile, because there is no way to invalidate the page cache's copy of
> that metadata.

If you closed the file and reopened it, then on NFS this would have this
effect I believe, as it needs to check whether other clients have modified
the file to give open/close coherency.

However, we are rather assuming the file isn't even open at the point it
is closed by the sender, which is what George is investigating.

If this isn't true, we have a problem anyway with (e.g.) emulated
devices which don't use O_DIRECT anyway. And I had thought (I may be
wrong) using O_DIRECT does not guarantee no read caching with NFS;
O_DIRECT merely guarantees the page cache is not used under Linux
and isn't defined under POSIX:
  http://www.spinics.net/lists/linux-nfs/msg17472.html

If it were just a write caching issue, we could use O_DSYNC instead of
O_DIRECT, which would at least ensure the copy from userspace.

-- 
Alex Bligh