linux-kernel - Re: [PATCH 26/94] Maple Tree: Add new data structure

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Message-ID: <YJ5wVncTFHHmY762@casper.infradead.org>
Date:   Fri, 14 May 2021 13:43:02 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Liam Howlett <liam.howlett@...cle.com>,
        "maple-tree@...ts.infradead.org" <maple-tree@...ts.infradead.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Song Liu <songliubraving@...com>,
        Davidlohr Bueso <dave@...olabs.net>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Laurent Dufour <ldufour@...ux.ibm.com>,
        David Rientjes <rientjes@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Rik van Riel <riel@...riel.com>,
        Michel Lespinasse <walken.cr@...il.com>
Subject: Re: [PATCH 26/94] Maple Tree: Add new data structure

On Fri, May 14, 2021 at 02:17:49PM +0200, Peter Zijlstra wrote:
> On Wed, Apr 28, 2021 at 03:36:02PM +0000, Liam Howlett wrote:
> > +static void mas_set_height(struct ma_state *mas)
> > +{
> > +	unsigned int new_flags = mas->tree->ma_flags;
> > +
> > +	new_flags &= ~MAPLE_HEIGHT_MASK;
> > +	new_flags |= mas->depth << MAPLE_HEIGHT_OFFSET;
> > +	mas->tree->ma_flags = new_flags;
> > +}
> 
> This goes sideways when mas->depth is too large; I didn't quickly see an
> assertion / range checks for mas->depth anywhere.

Not arguing that we shouldn't have such an assertion (to catch bugs),
but with 4 bits, that's a height 15 tree.  At a minimum fanout of 5.5
per level (with a minimum of 8.5 in the leaf), that's 197 x 10^9 entries.
At a more typical fanout of 8 per level and 12 entries in each leaf, it's
52.8 x 10^12 entries in the tree (about 2^46).  You'll run out of memory
and address space first.