Re: [PATCH 2/2] iomap: zero cached pages over unwritten extents on zero range

[Brian Foster <bfoster@redhat.com>]

On Mon, Oct 19, 2020 at 12:55:19PM -0400, Brian Foster wrote:
> On Thu, Oct 15, 2020 at 10:49:01AM +0100, Christoph Hellwig wrote:
> > > +iomap_zero_range_skip_uncached(struct inode *inode, loff_t *pos,
> > > +		loff_t *count, loff_t *written)
> > > +{
> > > +	unsigned dirty_offset, bytes = 0;
> > > +
> > > +	dirty_offset = page_cache_seek_hole_data(inode, *pos, *count,
> > > +				SEEK_DATA);
> > > +	if (dirty_offset == -ENOENT)
> > > +		bytes = *count;
> > > +	else if (dirty_offset > *pos)
> > > +		bytes = dirty_offset - *pos;
> > > +
> > > +	if (bytes) {
> > > +		*pos += bytes;
> > > +		*count -= bytes;
> > > +		*written += bytes;
> > > +	}
> > 
> > I find the calling conventions weird.  why not return bytes and
> > keep the increments/decrements of the three variables in the caller?
> > 
> 
> No particular reason. IIRC I had it both ways and just landed on this.
> I'd change it, but as mentioned in the patch 1 thread I don't think this
> patch is sufficient (with or without patch 1) anyways because the page
> can also have been reclaimed before we get here.
> 

Christoph,

What do you think about introducing behavior specific to
iomap_truncate_page() to unconditionally write zeroes over unwritten
extents? AFAICT that addresses the race and was historical XFS behavior
(via block_truncate_page()) before iomap, so is not without precedent.
What I'd probably do is bury the caller's did_zero parameter into a new
internal struct iomap_zero_data to pass down into
iomap_zero_range_actor(), then extend that structure with a
'zero_unwritten' field such that iomap_zero_range_actor() can do this:

        if (srcmap->type == IOMAP_HOLE ||
            (srcmap->type == IOMAP_UNWRITTEN && !zdata->zero_unwritten))
                return count;

iomap_truncate_page() would set that flag either via open coding
iomap_zero_range() or creating a new internal wrapper. Hm?

Brian

> Brian
>