On Tue, Oct 08, 2019 at 12:52:07PM +0200, Jan Kara wrote:
> On Thu 03-10-19 21:34:00, Matthew Bobrowski wrote:
> > This patch introduces a new direct I/O read path that makes use of the
> > iomap infrastructure.
> >
> > The new function ext4_dio_read_iter() is responsible for calling into
> > the iomap infrastructure via iomap_dio_rw(). If the read operation
> > being performed on the inode does not pass the preliminary checks
> > performed within ext4_dio_supported(), then we simply fallback to
> > buffered I/O in order to fulfil the request.
> >
> > Existing direct I/O read buffer_head code has been removed as it's now
> > redundant.
> >
> > Signed-off-by: Matthew Bobrowski <mbobrowski@xxxxxxxxxxxxxx>
>
> The patch looks good to me. Just one small nit below. With that fixed, you
> can add:
>
> Reviewed-by: Jan Kara <jack@xxxxxxx>
Cool, I'll fix it!
> > + /*
> > + * Get exclusion from truncate and other inode operations.
> > + */
> > + if (!inode_trylock_shared(inode)) {
> > + if (iocb->ki_flags & IOCB_NOWAIT)
> > + return -EAGAIN;
> > + inode_lock_shared(inode);
> > + }
>
> I've noticed here you actually introduce new trylock pattern - previously
> we had unconditional inode_lock_shared() in ext4_direct_IO_read(). So the
> cleanest would be to just use unconditional inode_lock_shared() here and
> then fixup IOCB_NOWAIT handling (I agree that was missing in the original
> code) in a separate patch.
Right, so I will just have an unconditional call to
inode_lock_shared() and in the patch that follows I will fix it up to
apply the new pattern.
> And the pattern should rather look like:
>
> if (iocb->ki_flags & IOCB_NOWAIT) {
> if (!inode_trylock_shared(inode))
> return -EAGAIN;
> } else {
> inode_lock_shared(inode);
> }
>
> to avoid two atomical operations instead of one in the fast path. No need
> to repeat old mistakes when we know better :).
Yes, also agree.
--<M>--
