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From:   David Wysochanski <dwysocha@redhat.com>
Date:   Mon, 10 Aug 2020 12:35:53 -0400
Message-ID: <CALF+zO=DkGmNDrrr-WxU6L1Xw8MA4+NrqVbvNMctwSKjy0Yh_w@mail.gmail.com>
Subject: Re: [GIT PULL] fscache rewrite -- please drop for now
To:     David Howells <dhowells@redhat.com>
Cc:     Steve French <smfrench@gmail.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Matthew Wilcox <willy@infradead.org>,
        Christoph Hellwig <hch@lst.de>,
        Jeff Layton <jlayton@redhat.com>,
        Trond Myklebust <trondmy@hammerspace.com>,
        Anna Schumaker <anna.schumaker@netapp.com>,
        Steve French <sfrench@samba.org>,
        Eric Van Hensbergen <ericvh@gmail.com>,
        linux-cachefs@redhat.com, linux-afs@lists.infradead.org,
        linux-nfs <linux-nfs@vger.kernel.org>,
        CIFS <linux-cifs@vger.kernel.org>, ceph-devel@vger.kernel.org,
        v9fs-developer@lists.sourceforge.net,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Mon, Aug 10, 2020 at 11:48 AM David Howells <dhowells@redhat.com> wrote:
>
> Steve French <smfrench@gmail.com> wrote:
>
> > cifs.ko also can set rsize quite small (even 1K for example, although
> > that will be more than 10x slower than the default 4MB so hopefully no
> > one is crazy enough to do that).
>
> You can set rsize < PAGE_SIZE?
>
> > I can't imagine an SMB3 server negotiating an rsize or wsize smaller than
> > 64K in today's world (and typical is 1MB to 8MB) but the user can specify a
> > much smaller rsize on mount.  If 64K is an adequate minimum, we could change
> > the cifs mount option parsing to require a certain minimum rsize if fscache
> > is selected.
>
> I've borrowed the 256K granule size used by various AFS implementations for
> the moment.  A 512-byte xattr can thus hold a bitmap covering 1G of file
> space.
>
>

Is it possible to make the granule size configurable, then reject a
registration if the size is too small or not a power of 2?  Then a
netfs using the API could try to set equal to rsize, and then error
out with a message if the registration was rejected.