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

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Date:   Tue, 15 Nov 2022 04:09:06 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     dwysocha@...hat.com, Rohith Surabattula <rohiths.msft@...il.com>,
        Steve French <sfrench@...ba.org>,
        Shyam Prasad N <nspmangalore@...il.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, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2] mm, netfs, fscache: Stop read optimisation when
 folio removed from pagecache

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?