Re: [PATCH v6 10/13] xfs: iomap COW-based atomic write support

[John Garry]

> > +		}
> >   		end_fsb = imap.br_startoff + imap.br_blockcount;
> >   		length = XFS_FSB_TO_B(mp, end_fsb) - offset;
> >   	}
> >   
> > -	if (imap_needs_alloc(inode, flags, &imap, nimaps))
> > +	needs_alloc = imap_needs_alloc(inode, flags, &imap, nimaps);
> > +
> > +	if (flags & IOMAP_ATOMIC) {
> > +		error = -EAGAIN;
> > +		/*
> > +		 * If we allocate less than what is required for the write
> > +		 * then we may end up with multiple mappings, which means that
> > +		 * REQ_ATOMIC-based cannot be used, so avoid this possibility.
> > +		 */
> > +		if (needs_alloc && orig_end_fsb - offset_fsb > 1)
> > +			goto out_unlock;
>
> I have a quick question here. Based on above check it looks like
> allocation requests on a hole or the 1st time allocation (append writes)
> for a given logical range will always be done using CoW fallback
> mechanism, isn't it? 

Right, but...


> So that means HW based multi-fsblock atomic write
> request will only happen for over writes (non-discontigous extent),
> correct?

For an unwritten pre-allocated extent, we can use the REQ_ATOMIC method.

fallocate (without ZERO RANGE) would give a pre-allocated unwritten 
extent, and a write there would not technically be an overwrite.

> Now, it's not always necessary that if we try to allocate an extent for
> the given range, it results into discontiguous extents. e.g. say, if the
> entire range being written to is a hole or append writes, then it might
> just allocate a single unwritten extent which is valid for doing an
> atomic write using HW/BIOs right?

Right

> And it is valid to write using unwritten extent as long as we don't have
> mixed mappings i.e. the entire range should either be unwritten or
> written for the atomic write to be untorned, correct?

We can't write to discontiguous extents, and a mixed mapping would mean 
discontiguous extents.

And, as mentioned earlier, it is ok to use REQ_ATOMIC method on an 
unwritten extent.

> I am guessing this is kept intentional?
Yes

Thanks,
John