Re: [Qemu-devel] Safely reopening image files by stashing fds

["Daniel P. Berrange" <berrange@redhat.com>]

On Fri, Aug 05, 2011 at 10:27:00AM +0100, Stefan Hajnoczi wrote:
> On Fri, Aug 5, 2011 at 10:04 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
> > On 08/05/2011 10:40 AM, Stefan Hajnoczi wrote:
> >>
> >> 4. Implement bdrv_reopen()
> >>
> >> First call bdrv_extract_fds() to stash the file descriptors, then close
> >> the
> >> block device.  Try opening the new image but if that fails, reopen using
> >> the
> >> stashed file descriptors.
> >
> > Why not do the latter unconditionally?
> 
> Because you cannot change O_DIRECT on an open fd :(.  This is why
> we're going through this pain.

Hmm, I remember hearing that before, but looking at the current fcntl()
manpage, it claims you *can* change O_DIRECT using SET_FL. Perhaps this
is a newish feature, but it'd be nicer to use it if possible ?

  [man 2 fcntl]
       F_SETFL (long)
              Set  the  file status flags to the value specified by arg.  File
              access mode (O_RDONLY, O_WRONLY, O_RDWR) and file creation flags
              (i.e.,  O_CREAT,  O_EXCL, O_NOCTTY, O_TRUNC) in arg are ignored.
              On Linux this command can only  change  the  O_APPEND,  O_ASYNC,
              O_DIRECT, O_NOATIME, and O_NONBLOCK flags.
  [/man]

Regards,
Daniel
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