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Date:   Wed, 10 Jan 2018 09:55:56 +1100
From:   Dave Chinner <david@...morbit.com>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        viro@...iv.linux.org.uk, linux-nfs@...r.kernel.org,
        bfields@...ldses.org, neilb@...e.de, jack@...e.de,
        linux-ext4@...r.kernel.org, tytso@....edu,
        adilger.kernel@...ger.ca, linux-xfs@...r.kernel.org,
        darrick.wong@...cle.com, linux-btrfs@...r.kernel.org, clm@...com,
        jbacik@...com, dsterba@...e.com, linux-integrity@...r.kernel.org,
        zohar@...ux.vnet.ibm.com, dmitry.kasatkin@...il.com,
        linux-afs@...ts.infradead.org, dhowells@...hat.com,
        jaltman@...istor.com, krzk@...nel.org
Subject: Re: [PATCH v5 19/19] fs: handle inode->i_version more efficiently

On Tue, Jan 09, 2018 at 09:10:59AM -0500, Jeff Layton wrote:
> From: Jeff Layton <jlayton@...hat.com>
> 
> Since i_version is mostly treated as an opaque value, we can exploit that
> fact to avoid incrementing it when no one is watching. With that change,
> we can avoid incrementing the counter on writes, unless someone has
> queried for it since it was last incremented. If the a/c/mtime don't
> change, and the i_version hasn't changed, then there's no need to dirty
> the inode metadata on a write.
> 
> Convert the i_version counter to an atomic64_t, and use the lowest order
> bit to hold a flag that will tell whether anyone has queried the value
> since it was last incremented.
> 
> When we go to maybe increment it, we fetch the value and check the flag
> bit.  If it's clear then we don't need to do anything if the update
> isn't being forced.
> 
> If we do need to update, then we increment the counter by 2, and clear
> the flag bit, and then use a CAS op to swap it into place. If that
> works, we return true. If it doesn't then do it again with the value
> that we fetch from the CAS operation.
> 
> On the query side, if the flag is already set, then we just shift the
> value down by 1 bit and return it. Otherwise, we set the flag in our
> on-stack value and again use cmpxchg to swap it into place if it hasn't
> changed. If it has, then we use the value from the cmpxchg as the new
> "old" value and try again.
> 
> This method allows us to avoid incrementing the counter on writes (and
> dirtying the metadata) under typical workloads. We only need to increment
> if it has been queried since it was last changed.
> 
> Signed-off-by: Jeff Layton <jlayton@...hat.com>
> Reviewed-by: Jan Kara <jack@...e.cz>

Documentation helps a lot in understanding all this. Thanks for
adding it into the patch!

Acked-by: Dave Chinner <dchinner@...hat.com>
-- 
Dave Chinner
david@...morbit.com

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