Re: [PATCH V2 2/2] ublk selftests: add --no_ublk_fixed_fd for not using registered ublk char device

[Ming Lei <ming.lei@redhat.com>]

On Mon, Aug 25, 2025 at 09:53:27PM -0700, Caleb Sander Mateos wrote:
> On Mon, Aug 25, 2025 at 5:49 AM Ming Lei <ming.lei@redhat.com> wrote:
> >
> > Add a new command line option --no_ublk_fixed_fd that excludes the ublk
> > control device (/dev/ublkcN) from io_uring's registered files array.
> > When this option is used, only backing files are registered starting
> > from index 1, while the ublk control device is accessed using its raw
> > file descriptor.
> >
> > Add ublk_get_registered_fd() helper function that returns the appropriate
> > file descriptor for use with io_uring operations, taking ublk_queue *
> > parameter instead of ublk_thread * for better performance.
> >
> > Key optimizations implemented:
> > - Cache UBLKS_Q_NO_UBLK_FIXED_FD flag in ublk_queue.flags to avoid
> >   reading dev->no_ublk_fixed_fd in fast path
> > - Cache ublk char device fd in ublk_queue.ublk_fd for fast access
> > - Update ublk_get_registered_fd() to use ublk_queue * parameter
> > - Update io_uring_prep_buf_register/unregister() to use ublk_queue *
> > - Replace ublk_device * access with ublk_queue * access in fast paths
> >
> > This improves performance by:
> > - Eliminating device structure traversal in hot paths
> > - Better cache locality with queue-local data access
> > - Reduced indirection for flag and fd lookups
> 
> Are you saying that performance is better when using the raw
> /dev/ublkcN fd instead of an io_uring registered file? That would be
> really surprising to me, since the whole point of io_uring file
> registration is to avoid the file reference counting in the I/O path.

No, here it just describes one implementation detail by caching per-device
flag(no_ublk_fixed_fd) in queue's flag.

The test can trigger hang with patch V1 by passing --no_ublk_fixed_fd
because /dev/ublkcN and io_uring closes can be depended, both are run
from task work context in current task.


Thanks,
Ming