linux-ext4 - Re: [man-pages RFC PATCH v4] statx, inode: document the new STATX_INO

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Date:   Tue, 13 Sep 2022 13:30:58 +1000
From:   "NeilBrown" <neilb@...e.de>
To:     "Dave Chinner" <david@...morbit.com>
Cc:     "Jeff Layton" <jlayton@...nel.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        "Theodore Ts'o" <tytso@....edu>, "Jan Kara" <jack@...e.cz>,
        adilger.kernel@...ger.ca, djwong@...nel.org,
        trondmy@...merspace.com, viro@...iv.linux.org.uk,
        zohar@...ux.ibm.com, xiubli@...hat.com, chuck.lever@...cle.com,
        lczerner@...hat.com, brauner@...nel.org, fweimer@...hat.com,
        linux-man@...r.kernel.org, linux-api@...r.kernel.org,
        linux-btrfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, ceph-devel@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-nfs@...r.kernel.org,
        linux-xfs@...r.kernel.org
Subject: Re: [man-pages RFC PATCH v4] statx, inode: document the new
 STATX_INO_VERSION field

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