linux-kernel - Re: [PATCH v4 02/28] mm: Add an unlock function for PG_private_2/PG

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Message-ID: <20210316233244.GE3420@casper.infradead.org>
Date:   Tue, 16 Mar 2021 23:32:44 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Trond Myklebust <trondmy@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Steve French <sfrench@...ba.org>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Christoph Hellwig <hch@....de>,
        Alexander Viro <viro@...iv.linux.org.uk>, linux-mm@...ck.org,
        linux-cachefs@...hat.com, linux-afs@...ts.infradead.org,
        linux-nfs@...r.kernel.org, linux-cifs@...r.kernel.org,
        ceph-devel@...r.kernel.org, v9fs-developer@...ts.sourceforge.net,
        linux-fsdevel@...r.kernel.org, Jeff Layton <jlayton@...hat.com>,
        David Wysochanski <dwysocha@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 02/28] mm: Add an unlock function for
 PG_private_2/PG_fscache

On Tue, Mar 16, 2021 at 08:38:00PM +0000, David Howells wrote:
> Matthew Wilcox <willy@...radead.org> wrote:
> 
> > So ... a page with both flags cleared should have a refcount of N.
> > A page with one or both flags set should have a refcount of N+1.
> > ...
> > How is a poor filesystem supposed to make that true?  Also btrfs has this
> > problem since it uses private_2 for its own purposes.
> 
> It's simpler if it's N+2 for both patches set.  Btw, patch 13 adds that - and
> possibly that should be merged into an earlier patch.

So ...

static inline int page_has_private(struct page *page)
{
	unsigned long flags = page->flags;
	return ((flags >> PG_private) & 1) + ((flags >> PG_private_2) & 1);
}

perhaps?