Re: [PATCHv11 8/8] iomap: Add per-block dirty state tracking to improve performance

[Matthew Wilcox <willy@infradead.org>]

On Fri, Jul 07, 2023 at 08:16:17AM +1000, Dave Chinner wrote:
> On Thu, Jul 06, 2023 at 06:42:36PM +0100, Matthew Wilcox wrote:
> > On Thu, Jul 06, 2023 at 08:16:05PM +0530, Ritesh Harjani wrote:
> > > > @@ -1645,6 +1766,11 @@ iomap_writepage_map(struct iomap_writepage_ctx *wpc,
> > > >  	int error = 0, count = 0, i;
> > > >  	LIST_HEAD(submit_list);
> > > >  
> > > > +	if (!ifs && nblocks > 1) {
> > > > +		ifs = ifs_alloc(inode, folio, 0);
> > > > +		iomap_set_range_dirty(folio, 0, folio_size(folio));
> > > > +	}
> > > > +
> > > >  	WARN_ON_ONCE(ifs && atomic_read(&ifs->write_bytes_pending) != 0);
> > > >  
> > > >  	/*
> > > > @@ -1653,7 +1779,7 @@ iomap_writepage_map(struct iomap_writepage_ctx *wpc,
> > > >  	 * invalid, grab a new one.
> > > >  	 */
> > > >  	for (i = 0; i < nblocks && pos < end_pos; i++, pos += len) {
> > > > -		if (ifs && !ifs_block_is_uptodate(ifs, i))
> > > > +		if (ifs && !ifs_block_is_dirty(folio, ifs, i))
> > > >  			continue;
> > > >  
> > > >  		error = wpc->ops->map_blocks(wpc, inode, pos);
> > > > @@ -1697,6 +1823,7 @@ iomap_writepage_map(struct iomap_writepage_ctx *wpc,
> > > >  		}
> > > >  	}
> > > >  
> > > > +	iomap_clear_range_dirty(folio, 0, end_pos - folio_pos(folio));
> > > >  	folio_start_writeback(folio);
> > > >  	folio_unlock(folio);
> > > >  
> > > 
> > > I think we should fold below change with this patch. 
> > > end_pos is calculated in iomap_do_writepage() such that it is either
> > > folio_pos(folio) + folio_size(folio), or if this value becomes more then
> > > isize, than end_pos is made isize.
> > > 
> > > The current patch does not have a functional problem I guess. But in
> > > some cases where truncate races with writeback, it will end up marking
> > > more bits & later doesn't clear those. Hence I think we should correct
> > > it using below diff.
> > 
> > I don't think this is the only place where we'll set dirty bits beyond
> > EOF.  For example, if we mmap the last partial folio in a file,
> > page_mkwrite will dirty the entire folio, but we won't write back
> > blocks past EOF.  I think we'd be better off clearing all the dirty
> > bits in the folio, even the ones past EOF.  What do you think?
> 
> Clear the dirty bits beyond EOF where we zero the data range beyond
> EOF in iomap_do_writepage() via folio_zero_segment()?

That would work, but I think it's simpler to change:

-	iomap_clear_range_dirty(folio, 0, end_pos - folio_pos(folio));
+	iomap_clear_range_dirty(folio, 0, folio_size(folio));