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From: Linus Torvalds <torvalds@linux-foundation.org>
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>
Subject: Re: [GIT PULL] fscache: I/O API modernisation and netfs helper library
Date: Tue, 9 Feb 2021 13:19:01 -0800	[thread overview]
Message-ID: <CAHk-=wh+2gbF7XEjYc=HV9w_2uVzVf7vs60BPz0gFA=+pUm3ww@mail.gmail.com> (raw)
In-Reply-To: <20210209202134.GA308988@casper.infradead.org>

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

  reply	other threads:[~2021-02-09 23:42 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-09 16:09 David Howells
2021-02-09 19:06 ` Linus Torvalds
2021-02-09 19:45   ` Jeff Layton
2021-02-09 20:21   ` Matthew Wilcox
2021-02-09 21:19     ` Linus Torvalds [this message]
2021-02-09 21:55     ` David Howells
2021-02-10 16:36     ` David Howells
2021-02-09 21:25   ` David Howells
2021-02-09 22:42   ` David Wysochanski
2021-02-09 21:10 ` David Howells
2021-02-10 16:29 ` David Howells
2021-02-10 16:33 ` David Howells
2021-02-10 20:43   ` Linus Torvalds
2021-02-11 22:38   ` David Howells
2021-02-11 23:20   ` David Howells
2021-02-12 16:40     ` David Wysochanski
2021-02-13  1:05     ` Linus Torvalds
2021-02-15  0:22     ` David Howells
2021-02-15  1:01       ` Linus Torvalds

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