Re: [PATCH v3 5/6] ext4: introduce direct IO write path using iomap infrastructure

[Christoph Hellwig <hch@infradead.org>]

On Tue, Sep 17, 2019 at 08:37:41AM +1000, Matthew Bobrowski wrote:
> > Independent of the error return issue you probably want to split
> > modifying ext4_write_checks into a separate preparation patch.
> 
> Providing that there's no objections to introducing a possible performance
> change with this separate preparation patch (overhead of calling
> file_remove_privs/file_update_time twice), then I have no issues in doing so.

Well, we should avoid calling it twice.  But what caught my eye is that
the buffered I/O path also called this function, so we are changing it as
well here.  If that actually is safe (I didn't review these bits carefully
and don't know ext4 that well) the overall refactoring of the write
flow might belong into a separate prep patch (that is not relying
on ->direct_IO, the checks changes, etc).

> > > +	if (!inode_trylock(inode)) {
> > > +		if (iocb->ki_flags & IOCB_NOWAIT)
> > > +			return -EAGAIN;
> > > +		inode_lock(inode);
> > > +	}
> > > +
> > > +	if (!ext4_dio_checks(inode)) {
> > > +		inode_unlock(inode);
> > > +		/*
> > > +		 * Fallback to buffered IO if the operation on the
> > > +		 * inode is not supported by direct IO.
> > > +		 */
> > > +		return ext4_buffered_write_iter(iocb, from);
> > 
> > I think you want to lift the locking into the caller of this function
> > so that you don't have to unlock and relock for the buffered write
> > fallback.
> 
> I don't exactly know what you really mean by "lift the locking into the caller
> of this function". I'm interpreting that as moving the inode_unlock()
> operation into ext4_buffered_write_iter(), but I can't see how that would be
> any different from doing it directly here? Wouldn't this also run the risk of
> the locks becoming unbalanced as we'd need to add checks around whether the
> resource is being contended? Maybe I'm misunderstanding something here...

With that I mean to acquire the inode lock in ext4_file_write_iter
instead of the low-level buffered I/O or direct I/O routines.