linux-kernel - Re: [PATCH] Fix panic in __d_lookup with high dentry hashtable counts

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Date:	Fri, 13 Jan 2012 10:36:42 -0600
From:	Dimitri Sivanich <sivanich@....com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	linux-kernel@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jiri Kosina <jkosina@...e.cz>, Avi Kivity <avi@...hat.com>,
	linux-fsdevel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH] Fix panic in __d_lookup with high dentry hashtable
 counts

On Fri, Jan 13, 2012 at 04:22:36PM +0000, Al Viro wrote:
> On Fri, Jan 13, 2012 at 09:52:37AM -0600, Dimitri Sivanich wrote:
> > When the number of dentry cache hash table entries gets too high
> > (2147483648 entries), use of a signed integer in the initialization
> > loop prevents the dentry_hashtable from getting initialized, resulting
> > in a panic in __d_lookup.  Fixing this in dcache_init and a few other
> > spots for consistency.
> 
> >  static void __init dcache_init(void)
> >  {
> > -	int loop;
> > +	long loop;
> 
> You've got to be kidding.  Note that D_HASHMASK is at most 32bit.  Use
> of long here is an overkill and so's 2^31 hash buckets (that's what,
> 16Gb in hash list heads alone?  What kind of average chain length do
> you expect, BTW?)

Yes, long might be overkill right now, but the code is all __init time code.
I don't have numbers showing average chain length at this point, I was
simply fixing this one end case

> 
> Can alloc_large_system_hash() produce the horrors that large, anyway?

On a 16TB system, alloc_large_system_hash() produces 2^31 hash buckets, yes.

Would simply capping the value in alloc_large_system_hash() be more palatable?

Something like the following?

Index: linux/mm/page_alloc.c
===================================================================
--- linux.orig/mm/page_alloc.c
+++ linux/mm/page_alloc.c
@@ -5257,6 +5257,7 @@ void *__init alloc_large_system_hash(con
        if (max == 0) {
                max = ((unsigned long long)nr_all_pages << PAGE_SHIFT) >> 4;
                do_div(max, bucketsize);
+               max = min(max, 1ULL << 30);
        }
 
        if (numentries > max)

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