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Date: Mon, 12 Sep 2022 14:56:33 +0000 From: Trond Myklebust <trondmy@...merspace.com> To: "bfields@...ldses.org" <bfields@...ldses.org> CC: "zohar@...ux.ibm.com" <zohar@...ux.ibm.com>, "djwong@...nel.org" <djwong@...nel.org>, "xiubli@...hat.com" <xiubli@...hat.com>, "brauner@...nel.org" <brauner@...nel.org>, "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>, "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>, "neilb@...e.de" <neilb@...e.de>, "david@...morbit.com" <david@...morbit.com>, "fweimer@...hat.com" <fweimer@...hat.com>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "jlayton@...nel.org" <jlayton@...nel.org>, "chuck.lever@...cle.com" <chuck.lever@...cle.com>, "linux-man@...r.kernel.org" <linux-man@...r.kernel.org>, "linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>, "tytso@....edu" <tytso@....edu>, "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>, "jack@...e.cz" <jack@...e.cz>, "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>, "linux-btrfs@...r.kernel.org" <linux-btrfs@...r.kernel.org>, "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>, "adilger.kernel@...ger.ca" <adilger.kernel@...ger.ca>, "lczerner@...hat.com" <lczerner@...hat.com>, "ceph-devel@...r.kernel.org" <ceph-devel@...r.kernel.org> Subject: Re: [man-pages RFC PATCH v4] statx, inode: document the new STATX_INO_VERSION field On Mon, 2022-09-12 at 10:50 -0400, J. Bruce Fields wrote: > On Mon, Sep 12, 2022 at 02:15:16PM +0000, Trond Myklebust wrote: > > On Mon, 2022-09-12 at 09:51 -0400, J. Bruce Fields wrote: > > > On Mon, Sep 12, 2022 at 08:55:04AM -0400, Jeff Layton wrote: > > > > Because of the "seen" flag, we have a 63 bit counter to play > > > > with. > > > > Could > > > > we use a similar scheme to the one we use to handle when > > > > "jiffies" > > > > wraps? Assume that we'd never compare two values that were more > > > > than > > > > 2^62 apart? We could add i_version_before/i_version_after > > > > macros to > > > > make > > > > it simple to handle this. > > > > > > As far as I recall the protocol just assumes it can never wrap. > > > I > > > guess > > > you could add a new change_attr_type that works the way you > > > describe. > > > But without some new protocol clients aren't going to know what > > > to do > > > with a change attribute that wraps. > > > > > > I think this just needs to be designed so that wrapping is > > > impossible > > > in > > > any realistic scenario. I feel like that's doable? > > > > > > If we feel we have to catch that case, the only 100% correct > > > behavior > > > would probably be to make the filesystem readonly. > > > > > > > Which protocol? If you're talking about basic NFSv4, it doesn't > > assume > > anything about the change attribute and wrapping. > > > > The NFSv4.2 protocol did introduce the optional attribute > > 'change_attr_type' that tries to describe the change attribute > > behaviour to the client. It tells you if the behaviour is > > monotonically > > increasing, but doesn't say anything about the behaviour when the > > attribute value overflows. > > > > That said, the Linux NFSv4.2 client, which uses that > > change_attr_type > > attribute does deal with overflow by assuming standard uint64_t > > wrap > > around rules. i.e. it assumes bit values > 63 are truncated, > > meaning > > that the value obtained by incrementing (2^64-1) is 0. > > Yeah, it was the MONOTONIC_INCRE case I was thinking of. That's > interesting, I didn't know the client did that. > If you look at where we compare version numbers, it is always some variant of the following: static int nfs_inode_attrs_cmp_monotonic(const struct nfs_fattr *fattr, const struct inode *inode) { s64 diff = fattr->change_attr - inode_peek_iversion_raw(inode); if (diff > 0) return 1; return diff == 0 ? 0 : -1; } i.e. we do an unsigned 64-bit subtraction, and then cast it to the signed 64-bit equivalent in order to figure out which is the more recent value. -- Trond Myklebust Linux NFS client maintainer, Hammerspace trond.myklebust@...merspace.com
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