linux-kernel - Re: [PATCH 06/18] fsinfo: Add a uniquifier ID to struct mount [ver #21]

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Message-ID: <20200805193303.GM23808@casper.infradead.org>
Date:   Wed, 5 Aug 2020 20:33:03 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Miklos Szeredi <miklos@...redi.hu>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ian Kent <raven@...maw.net>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Christian Brauner <christian@...uner.io>,
        Jann Horn <jannh@...gle.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Karel Zak <kzak@...hat.com>, Jeff Layton <jlayton@...hat.com>,
        Linux API <linux-api@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org,
        LSM <linux-security-module@...r.kernel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 06/18] fsinfo: Add a uniquifier ID to struct mount [ver
 #21]

On Wed, Aug 05, 2020 at 04:30:10PM +0100, David Howells wrote:
> Miklos Szeredi <miklos@...redi.hu> wrote:
> 
> > idr_alloc_cyclic() seems to be a good template for doing the lower
> > 32bit allocation, and we can add code to increment the high 32bit on
> > wraparound.
> > 
> > Lots of code uses idr_alloc_cyclic() so I guess it shouldn't be too
> > bad in terms of memory use or performance.
> 
> It's optimised for shortness of path and trades memory for performance.  It's
> currently implemented using an xarray, so memory usage is dependent on the
> sparseness of the tree.  Each node in the tree is 576 bytes and in the worst
> case, each one node will contain one mount - and then you have to backfill the
> ancestry, though for lower memory costs.
> 
> Systemd makes life more interesting since it sets up a whole load of
> propagations.  Each mount you make may cause several others to be created, but
> that would likely make the tree more efficient.

I would recommend using xa_alloc and ignoring the ID assigned from
xa_alloc.  Looking up by unique ID is then a matter of iterating every
mount (xa_for_each()) looking for a matching unique ID in the mount
struct.  That's O(n) search, but it's faster than a linked list, and we
don't have that many mounts in a system.

The maple tree will handle this case more effectively, but I can't
recommend waiting for that to be ready.