Re: [PATCH v5 3/3] mm: Make filemap_release_folio() better inform shrink_folio_list()

From: Christoph Hellwig <hch@infradead.org>
To: David Howells <dhowells@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Steve French <sfrench@samba.org>,
	Shyam Prasad N <nspmangalore@gmail.com>,
	Rohith Surabattula <rohiths.msft@gmail.com>,
	Dave Wysochanski <dwysocha@redhat.com>,
	Dominique Martinet <asmadeus@codewreck.org>,
	Ilya Dryomov <idryomov@gmail.com>,
	linux-cachefs@redhat.com, linux-cifs@vger.kernel.org,
	linux-afs@lists.infradead.org,
	v9fs-developer@lists.sourceforge.net, ceph-devel@vger.kernel.org,
	linux-nfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-mm@kvack.org, Jeff Layton <jlayton@kernel.org>,
	linux-erofs@lists.ozlabs.org, linux-ext4@lists.ozlabs.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v5 3/3] mm: Make filemap_release_folio() better inform shrink_folio_list()
Date: Fri, 23 Dec 2022 07:31:14 -0800	[thread overview]
Message-ID: <Y6XJwvjKyTgRIiI3@infradead.org> (raw)
In-Reply-To: <167172134962.2334525.570622889806603086.stgit@warthog.procyon.org.uk>

On Thu, Dec 22, 2022 at 03:02:29PM +0000, David Howells wrote:
> Make filemap_release_folio() return one of three values:
> 
>  (0) FILEMAP_CANT_RELEASE_FOLIO
> 
>      Couldn't release the folio's private data, so the folio can't itself
>      be released.
> 
>  (1) FILEMAP_RELEASED_FOLIO
> 
>      The private data on the folio was released and the folio can be
>      released.
> 
>  (2) FILEMAP_FOLIO_HAD_NO_PRIVATE

These names read really odd, due to the different placementments
of FOLIO, the present vs past tense and the fact that 2 also released
the folio, and the reliance of callers that one value of an enum
must be 0, while no unprecedented, is a bit ugly.

But do we even need them?  What abut just open coding
filemap_release_folio (which is a mostly trivial function) in
shrink_folio_list, which is the only place that cares?

	if (folio_has_private(folio) && folio_needs_release(folio)) {
		if (folio_test_writeback(folio))
			goto activate_locked;

		if (mapping && mapping->a_ops->release_folio) {
			if (!mapping->a_ops->release_folio(folio, gfp))
				goto activate_locked;
		} else {
			if (!try_to_free_buffers(folio))
				goto activate_locked;
		}

		if (!mapping && folio_ref_count(folio) == 1) {
			...

alternatively just keep using filemap_release_folio and just add the
folio_needs_release in the first branch.  That duplicates the test,
but makes the change a one-liner.