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

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Message-ID: <Y7mKZj/RnD2aW5jU@casper.infradead.org>
Date:   Sat, 7 Jan 2023 15:06:14 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     David Howells <dhowells@...hat.com>,
        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 Fri, Dec 23, 2022 at 07:31:14AM -0800, Christoph Hellwig wrote:
> 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.

Agreed.  The thing is that it's not the filemap that's being released,
it's the folio.  So these should be:

	FOLIO_RELEASE_SUCCESS
	FOLIO_RELEASE_FAILED
	FOLIO_RELEASE_NO_PRIVATE

... but of course, NO_PRIVATE is also a success.  So it's a really weird
thing to be reporting.  I'm with you on the latter half of this email:

> 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.

Or just drop patch 3 entirely?