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

[Paolo Bonzini]

On 08/05/2011 11:27 AM, Stefan Hajnoczi wrote:
> > > 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.
>
> The only method I've found that works is to open("/proc/self/fd/X",
> new_flags) but that's non-portable.

Maybe I'm missing something obvious, but so is O_DIRECT, no? :)

So for Linux you can dup the stashed file descriptors using 
/proc/self/fd and change flags directly, and for other OSes you can dup 
them using dup2 and change flags with F_SETFL.  In any case, reopening 
can always be done using the stashed descriptors (or BlockDriverStates).

Paolo