linux-kernel - Re: [GIT PULL] fscache: I/O API modernisation and netfs helper library

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAHk-=wh+2gbF7XEjYc=HV9w_2uVzVf7vs60BPz0gFA=+pUm3ww@mail.gmail.com>
Date:   Tue, 9 Feb 2021 13:19:01 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     David Howells <dhowells@...hat.com>,
        Jeff Layton <jlayton@...hat.com>,
        David Wysochanski <dwysocha@...hat.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Trond Myklebust <trondmy@...merspace.com>,
        Steve French <sfrench@...ba.org>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        ceph-devel@...r.kernel.org, linux-afs@...ts.infradead.org,
        linux-cachefs@...hat.com, CIFS <linux-cifs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        "open list:NFS, SUNRPC, AND..." <linux-nfs@...r.kernel.org>,
        v9fs-developer@...ts.sourceforge.net,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [GIT PULL] fscache: I/O API modernisation and netfs helper library

On Tue, Feb 9, 2021 at 12:21 PM Matthew Wilcox <willy@...radead.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