linux-kernel - Re: [PATCH v5 3/3] mm: Make filemap_release_folio() better inform shrink_folio

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Message-ID: <Y6XJwvjKyTgRIiI3@infradead.org>
Date:   Fri, 23 Dec 2022 07:31:14 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Steve French <sfrench@...ba.org>,
        Shyam Prasad N <nspmangalore@...il.com>,
        Rohith Surabattula <rohiths.msft@...il.com>,
        Dave Wysochanski <dwysocha@...hat.com>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Ilya Dryomov <idryomov@...il.com>, linux-cachefs@...hat.com,
        linux-cifs@...r.kernel.org, linux-afs@...ts.infradead.org,
        v9fs-developer@...ts.sourceforge.net, ceph-devel@...r.kernel.org,
        linux-nfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, Jeff Layton <jlayton@...nel.org>,
        linux-erofs@...ts.ozlabs.org, linux-ext4@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 3/3] mm: Make filemap_release_folio() better inform
 shrink_folio_list()

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.