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From:   "NeilBrown" <neilb@suse.de>
To:     "Dave Chinner" <david@fromorbit.com>
Cc:     "Jeff Layton" <jlayton@kernel.org>,
        "J. Bruce Fields" <bfields@fieldses.org>,
        "Theodore Ts'o" <tytso@mit.edu>, "Jan Kara" <jack@suse.cz>,
        adilger.kernel@dilger.ca, djwong@kernel.org,
        trondmy@hammerspace.com, viro@zeniv.linux.org.uk,
        zohar@linux.ibm.com, xiubli@redhat.com, chuck.lever@oracle.com,
        lczerner@redhat.com, brauner@kernel.org, fweimer@redhat.com,
        linux-man@vger.kernel.org, linux-api@vger.kernel.org,
        linux-btrfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, ceph-devel@vger.kernel.org,
        linux-ext4@vger.kernel.org, linux-nfs@vger.kernel.org,
        linux-xfs@vger.kernel.org
Subject: Re: [man-pages RFC PATCH v4] statx, inode: document the new
 STATX_INO_VERSION field
In-reply-to: <20220913024109.GF3600936@dread.disaster.area>
References: <20220908155605.GD8951@fieldses.org>,
 <9e06c506fd6b3e3118da0ec24276e85ea3ee45a1.camel@kernel.org>,
 <20220908182252.GA18939@fieldses.org>,
 <44efe219dbf511492b21a653905448d43d0f3363.camel@kernel.org>,
 <20220909154506.GB5674@fieldses.org>,
 <125df688dbebaf06478b0911e76e228e910b04b3.camel@kernel.org>,
 <20220910145600.GA347@fieldses.org>,
 <9eaed9a47d1aef11fee95f0079e302bc776bc7ff.camel@kernel.org>,
 <20220913004146.GD3600936@dread.disaster.area>,
 <166303374350.30452.17386582960615006566@noble.neil.brown.name>,
 <20220913024109.GF3600936@dread.disaster.area>
Date:   Tue, 13 Sep 2022 13:30:58 +1000
Message-id: <166303985824.30452.7333958999671590160@noble.neil.brown.name>
Precedence: bulk

On Tue, 13 Sep 2022, Dave Chinner wrote:
> 
> Indeed, we know there are many systems out there that mount a
> filesystem, preallocate and map the blocks that are allocated to a
> large file, unmount the filesysetm, mmap the ranges of the block
> device and pass them to RDMA hardware, then have sensor arrays rdma
> data directly into the block device. Then when the measurement
> application is done they walk the ondisk metadata to remove the
> unwritten flags on the extents, mount the filesystem again and
> export the file data to a HPC cluster for post-processing.....

And this tool doesn't update the i_version?  Sounds like a bug.

> 
> So how does the filesystem know whether data the storage contains
> for it's files has been modified while it is unmounted and so needs
> to change the salt?

How does it know that no data is modified while it *is* mounted?  Some
assumptions have to be made.

> 
> The short answer is that it can't, and so we cannot make assumptions
> that a unmount/mount cycle has not changed the filesystem in any
> way....

If a mount-count is the best that XFS can do, then that is certainly
what it should use.

Thanks,
NeilBrown