Re: [PATCH 04/23] page-writeback: Convert write_cache_pages() to use filemap_get_folios_tag()

[Vishal Moola]

On Wed, Oct 19, 2022 at 08:01:52AM +1100, Dave Chinner wrote:
> On Thu, Sep 01, 2022 at 03:01:19PM -0700, Vishal Moola (Oracle) wrote:
> > Converted function to use folios throughout. This is in preparation for
> > the removal of find_get_pages_range_tag().
> > 
> > Signed-off-by: Vishal Moola (Oracle) <vishal.moola@xxxxxxxxx>
> > ---
> >  mm/page-writeback.c | 44 +++++++++++++++++++++++---------------------
> >  1 file changed, 23 insertions(+), 21 deletions(-)
> > 
> > diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> > index 032a7bf8d259..087165357a5a 100644
> > --- a/mm/page-writeback.c
> > +++ b/mm/page-writeback.c
> > @@ -2285,15 +2285,15 @@ int write_cache_pages(struct address_space *mapping,
> >  	int ret = 0;
> >  	int done = 0;
> >  	int error;
> > -	struct pagevec pvec;
> > -	int nr_pages;
> > +	struct folio_batch fbatch;
> > +	int nr_folios;
> >  	pgoff_t index;
> >  	pgoff_t end;		/* Inclusive */
> >  	pgoff_t done_index;
> >  	int range_whole = 0;
> >  	xa_mark_t tag;
> >  
> > -	pagevec_init(&pvec);
> > +	folio_batch_init(&fbatch);
> >  	if (wbc->range_cyclic) {
> >  		index = mapping->writeback_index; /* prev offset */
> >  		end = -1;
> > @@ -2313,17 +2313,18 @@ int write_cache_pages(struct address_space *mapping,
> >  	while (!done && (index <= end)) {
> >  		int i;
> >  
> > -		nr_pages = pagevec_lookup_range_tag(&pvec, mapping, &index, end,
> > -				tag);
> > -		if (nr_pages == 0)
> > +		nr_folios = filemap_get_folios_tag(mapping, &index, end,
> > +				tag, &fbatch);
> 
> This can find and return dirty multi-page folios if the filesystem
> enables them in the mapping at instantiation time, right?

Yup, it will.

> > +
> > +		if (nr_folios == 0)
> >  			break;
> >  
> > -		for (i = 0; i < nr_pages; i++) {
> > -			struct page *page = pvec.pages[i];
> > +		for (i = 0; i < nr_folios; i++) {
> > +			struct folio *folio = fbatch.folios[i];
> >  
> > -			done_index = page->index;
> > +			done_index = folio->index;
> >  
> > -			lock_page(page);
> > +			folio_lock(folio);
> >  
> >  			/*
> >  			 * Page truncated or invalidated. We can freely skip it
> > @@ -2333,30 +2334,30 @@ int write_cache_pages(struct address_space *mapping,
> >  			 * even if there is now a new, dirty page at the same
> >  			 * pagecache address.
> >  			 */
> > -			if (unlikely(page->mapping != mapping)) {
> > +			if (unlikely(folio->mapping != mapping)) {
> >  continue_unlock:
> > -				unlock_page(page);
> > +				folio_unlock(folio);
> >  				continue;
> >  			}
> >  
> > -			if (!PageDirty(page)) {
> > +			if (!folio_test_dirty(folio)) {
> >  				/* someone wrote it for us */
> >  				goto continue_unlock;
> >  			}
> >  
> > -			if (PageWriteback(page)) {
> > +			if (folio_test_writeback(folio)) {
> >  				if (wbc->sync_mode != WB_SYNC_NONE)
> > -					wait_on_page_writeback(page);
> > +					folio_wait_writeback(folio);
> >  				else
> >  					goto continue_unlock;
> >  			}
> >  
> > -			BUG_ON(PageWriteback(page));
> > -			if (!clear_page_dirty_for_io(page))
> > +			BUG_ON(folio_test_writeback(folio));
> > +			if (!folio_clear_dirty_for_io(folio))
> >  				goto continue_unlock;
> >  
> >  			trace_wbc_writepage(wbc, inode_to_bdi(mapping->host));
> > -			error = (*writepage)(page, wbc, data);
> > +			error = writepage(&folio->page, wbc, data);
> 
> Yet, IIUC, this treats all folios as if they are single page folios.
> i.e. it passes the head page of a multi-page folio to a callback
> that will treat it as a single PAGE_SIZE page, because that's all
> the writepage callbacks are currently expected to be passed...
> 
> So won't this break writeback of dirty multipage folios?

Yes, it appears it would. But it wouldn't because its already 'broken'.

The current find_get_pages_range_tag() actually has the exact same
issue. The current code to fill up the pages array is:

		pages[ret] = &folio->page;
		if (++ret == nr_pages) {
			*index = folio->index + folio_nr_pages(folio);
			goto out;

which behaves the same way as the issue you pointed out (both break
large folios). When I spoke to Matthew about this earlier, we decided
to go ahead with replacing the function and leave it up to the callers
to fix/handle large folios when the filesystem gets to it.

Its not great to leave it 'broken' but its something that isn't - or at
least shouldn't be - creating any problems at present. And I believe Matthew
has plans to address them at some point before they actually become problems?