Re: [RFC PATCH v2] mm, netfs, fscache: Stop read optimisation when folio removed from pagecache

From: Matthew Wilcox <willy@infradead.org>
To: David Howells <dhowells@redhat.com>
Cc: dwysocha@redhat.com, Rohith Surabattula <rohiths.msft@gmail.com>,
	Steve French <sfrench@samba.org>,
	Shyam Prasad N <nspmangalore@gmail.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, linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH v2] mm, netfs, fscache: Stop read optimisation when folio removed from pagecache
Date: Tue, 15 Nov 2022 04:09:06 +0000	[thread overview]
Message-ID: <Y3MQ4l1AJOgniprT@casper.infradead.org> (raw)
In-Reply-To: <166844174069.1124521.10890506360974169994.stgit@warthog.procyon.org.uk>

On Mon, Nov 14, 2022 at 04:02:20PM +0000, David Howells wrote:
> +++ b/mm/filemap.c
> @@ -3941,6 +3941,10 @@ bool filemap_release_folio(struct folio *folio, gfp_t gfp)
>  	struct address_space * const mapping = folio->mapping;
>  
>  	BUG_ON(!folio_test_locked(folio));
> +	if ((!mapping || !mapping_release_always(mapping))
> +	    && !folio_test_private(folio) &&
> +	    !folio_test_private_2(folio))
> +		return true;

Why do you need to test 'mapping' here?  Also this is the most
inconsistent style ...

	if ((!mapping || !mapping_release_always(mapping)) &&
	    !folio_test_private(folio) && !folio_test_private_2(folio))

works fine, but if you insist on splitting over three lines, then:

	if ((!mapping || !mapping_release_always(mapping)) &&
	    !folio_test_private(folio) && 
	    !folio_test_private_2(folio))

> @@ -276,7 +275,7 @@ static long mapping_evict_folio(struct address_space *mapping,
>  	if (folio_ref_count(folio) >
>  			folio_nr_pages(folio) + folio_has_private(folio) + 1)

I think this line is incorrect, right?  You don't increment the folio
refcount just because the folio has private2 set, do you?

>  		return 0;
> -	if (folio_has_private(folio) && !filemap_release_folio(folio, 0))
> +	if (!filemap_release_folio(folio, 0))
>  		return 0;
>  
>  	return remove_mapping(mapping, folio);

Can we get rid of folio_has_private() / page_has_private() now?