Re: [PATCH v9 1/6] mm: take a folio off WB_DONTCACHE_DIRTY only if it was counted there

[Christoph Hellwig <hch@lst.de>]

Please write a concise commit log.  This is a long meandering thought
of eletronic dreams making no sense.

> +/*
> + * Clear the uncached flag of a folio a cached lookup found, and take the
> + * folio off WB_DONTCACHE_DIRTY if it is counted there.
> + *
> + * folio_account_dirtied() counts a dropbehind folio under the i_pages lock,
> + * in the section that sets its dirty tag, after PG_dirty has been set
> + * without the folio lock.  Every path that uncounts a dirty folio holds the
> + * folio lock.  So with the folio locked and the i_pages lock held, the folio
> + * is counted when both PG_dirty and the dirty tag are set.  PG_dirty alone
> + * is not enough: a dirtier past folio_test_set_dirty() but not yet counting
> + * finds the flag clear and does not count the folio.  The tag alone is not
> + * enough either: __folio_cancel_dirty() uncounts a folio and clears
> + * PG_dirty, but leaves the tag until writeback or truncation clears it.  A
> + * folio another task has locked keeps the flag, as it would have had this
> + * lookup not run.
> + */

And this comment isn't much better.

> -	if (!(fgp_flags & FGP_DONTCACHE) && folio_test_clear_dropbehind(folio)) {
> -		if (folio_test_dirty(folio) &&
> -		    mapping_can_writeback(mapping)) {
> -			struct inode *inode = mapping->host;
> -			struct bdi_writeback *wb;
> -			struct wb_lock_cookie cookie = {};
> -			long nr = folio_nr_pages(folio);
> -
> -			wb = unlocked_inode_to_wb_begin(inode, &cookie);
> -			wb_stat_mod(wb, WB_DONTCACHE_DIRTY, -nr);
> -			unlocked_inode_to_wb_end(inode, &cookie);
> -		}
> -	}
> +	if (!(fgp_flags & FGP_DONTCACHE) && folio_test_dropbehind(folio))
> +		filemap_clear_dropbehind(mapping, folio, fgp_flags & FGP_LOCK);

Please don't mix code moves and non-trivial behavior changes.

Add a prep patch moving the code, and only then modify it.