Re: [PATCH] ext4: cancel dirty accounting for folios without buffers

[Zhang Yi]

On 6/24/2026 8:32 PM, Jan Kara wrote:
> On Wed 24-06-26 17:52:06, Zhu Jia wrote:
> > Hi Yi,
> >
> > Thanks for taking a look.
> >
> > Yes, clearing PAGECACHE_TAG_DIRTY/TOWRITE would make the page-cache state
> > cleaner. I had a version that did this by adding a helper around
> > folio_cancel_dirty() and clearing the xarray tags after confirming the
> > folio was still the same clean page-cache entry.
> >
> > It looked like this:
> >
> > static void ext4_cancel_dirty_folio(struct address_space *mapping,
> > 				    struct folio *folio)
> > {
> > 	XA_STATE(xas, &mapping->i_pages, folio->index);
> > 	unsigned long flags;
> >
> > 	folio_cancel_dirty(folio);
> >
> > 	xas_lock_irqsave(&xas, flags);
> > 	if (xas_load(&xas) == folio && !folio_test_dirty(folio)) {
> > 		xas_clear_mark(&xas, PAGECACHE_TAG_DIRTY);
> > 		xas_clear_mark(&xas, PAGECACHE_TAG_TOWRITE);
> > 	}
> > 	xas_unlock_irqrestore(&xas, flags);
> > }
> >
> > The reason I left the tags unchanged in this version is that I was not sure
> > whether it is appropriate for ext4 to open-code xarray tag cleanup directly.
> >
> > If you think this is the right direction, I can add the helper back and
> > send a v2.
>
> That was a good judgement! Playing with xarray tags like this in filesystem
> code is certainly not a good thing. For now, I'd leave the xarray tags
> dangling - they will be eventually synced with reality on next writeback
> attempt. If this inconsistency of tags needs to be fixed, the fix belongs
> to the generic code (so that it can be used in other places as well).
>
> 								Honza

Yes, I agree. Directly clearing the tag via open code is not a good
approach. However, I took a look at the !nr_to_submit branch in
ext4_bio_write_folio(), and it seems to have a similar simple handling
pattern—it directly calls __folio_start_writeback() and
folio_end_writeback(), which appears to be an elegant way to clear them.
Could we also call these two helpers just after folio_cancel_dirty()
here?

Thanks,
Yi.