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

[Matthew Wilcox <willy@infradead.org>]

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?