Re: [PATCH 07/19] iomap: use a srcmap for a read-modify-write I/O

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On Tue, Sep 10, 2019 at 04:39:59PM +0200, hch@xxxxxx wrote:
> On Tue, Sep 10, 2019 at 12:48:14PM +0000, Goldwyn Rodrigues wrote:
> > > +	if (srcmap.type)
> > > +		end = min(end, srcmap.offset + srcmap.length);
> > > +	if (pos + length > end)
> > > +		length = end - pos;
> > >  
> > 
> > 
> > Yes, that looks more correct. However, can we be smart and not bother
> > setting the minimum of end and srcmap.offset + srcmap.length if it is
> > not required? ie in situations where end coincides with block boundary.
> >  Or if srcmap.length falls short, until the last block boundary of
> > iomap? 
> > 
> > I did think about this scenario. This case is specific to CoW and
> > thought this is best handled by filesystem's iomap_begin(). If this
> > goes in, the filesystems would have to "falsify" srcmap length
> > information to maximize the amount of I/O that goes in one iteration.
> 
> The problem is really that we can easily run over the srcmap (that's
> what happened to me with XFS..)  One thing you've done in btrfs that
> I haven't done yet in XFS is to simply not bother with filling out
> the srcmap if we don't need to (that is if the iteration is fully
> page aligned in your patch set - the unshare op in this series will
> complicate things a little).  With that optimization the only case
> where the shortening of the iteration that matters is if the start
> is unaligned and needs a read-modify-write cycle, but the end is
> aligned and beyond the end of the srcmap.  Is that such an important
> case?

<shrug> You might as well, since it's still going to be a few weeks
until I carve off a iomap work branch for 5.5... :)

(He says stumbling in from vacation with half a brain)

--D

> > 
> > -- 
> > Goldwyn---end quoted text---



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