Re: [PATCH v2 13/18] monitor: fdset: Match against O_DIRECT

[Peter Xu <peterx@redhat.com>]

On Thu, May 23, 2024 at 04:05:43PM -0300, Fabiano Rosas wrote:
> We're about to enable the use of O_DIRECT in the migration code and
> due to the alignment restrictions imposed by filesystems we need to
> make sure the flag is only used when doing aligned IO.
> 
> The migration will do parallel IO to different regions of a file, so
> we need to use more than one file descriptor. Those cannot be obtained
> by duplicating (dup()) since duplicated file descriptors share the
> file status flags, including O_DIRECT. If one migration channel does
> unaligned IO while another sets O_DIRECT to do aligned IO, the
> filesystem would fail the unaligned operation.
> 
> The add-fd QMP command along with the fdset code are specifically
> designed to allow the user to pass a set of file descriptors with
> different access flags into QEMU to be later fetched by code that
> needs to alternate between those flags when doing IO.
> 
> Extend the fdset matching to behave the same with the O_DIRECT flag.
> 
> Signed-off-by: Fabiano Rosas <farosas@suse.de>

Reviewed-by: Peter Xu <peterx@redhat.com>

One thing I am confused but totally irrelevant to this specific change.

I wonder why do we need dupfds at all, and why client needs to remove-fd at
all.

It's about what would go wrong if qmp client only add-fd, then if it's
consumed by anything, it gets moved from "fds" list to "dupfds" list.  The
thing is I don't expect the client should pass over any fd that will not be
consumed.  Then if it's always consumed, why bother dup() at all, and why
bother an explicit remove-fd.

-- 
Peter Xu