Received-SPF: pass (google.com: domain of linux-nfs-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <591237.1612886997@warthog.procyon.org.uk> <CAHk-=wj-k86FOqAVQ4ScnBkX3YEKuMzqTEB2vixdHgovJpHc9w@mail.gmail.com>
 <20210209202134.GA308988@casper.infradead.org>
In-Reply-To: <20210209202134.GA308988@casper.infradead.org>
From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 9 Feb 2021 13:19:01 -0800
Message-ID: <CAHk-=wh+2gbF7XEjYc=HV9w_2uVzVf7vs60BPz0gFA=+pUm3ww@mail.gmail.com>
Subject: Re: [GIT PULL] fscache: I/O API modernisation and netfs helper library
To:     Matthew Wilcox <willy@infradead.org>
Cc:     David Howells <dhowells@redhat.com>,
        Jeff Layton <jlayton@redhat.com>,
        David Wysochanski <dwysocha@redhat.com>,
        Anna Schumaker <anna.schumaker@netapp.com>,
        Trond Myklebust <trondmy@hammerspace.com>,
        Steve French <sfrench@samba.org>,
        Dominique Martinet <asmadeus@codewreck.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        ceph-devel@vger.kernel.org, linux-afs@lists.infradead.org,
        linux-cachefs@redhat.com, CIFS <linux-cifs@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        "open list:NFS, SUNRPC, AND..." <linux-nfs@vger.kernel.org>,
        v9fs-developer@lists.sourceforge.net,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Tue, Feb 9, 2021 at 12:21 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> Yeah, I have trouble with the private2 vs fscache bit too.  I've been
> trying to persuade David that he doesn't actually need an fscache
> bit at all; he can just increment the page's refcount to prevent it
> from being freed while he writes data to the cache.

Does the code not hold a refcount already?

Honestly, the fact that writeback doesn't take a refcount, and then
has magic "if writeback is set, don't free" code in other parts of the
VM layer has been a problem already, when the wakeup ended up
"leaking" from a previous page to a new allocation.

I very much hope the fscache bit does not make similar mistakes,
because the rest of the VM will _not_ have special "if fscache is set,
then we won't do X" the way we do for writeback.

So I think the fscache code needs to hold a refcount regardless, and
that the fscache bit is set the page has to have a reference.

So what are the current lifetime rules for the fscache bit?

             Linus