Re: [PATCH] sctp: Fix port hash table size computation

[David Miller <davem@davemloft.net>]

From: Neil Horman <nhorman@tuxdriver.com>
Date: Thu, 18 Feb 2016 10:02:04 -0500

> Dmitry Vyukov noted recently that the sctp_port_hashtable had an error in
> its size computation, observing that the current method never guaranteed
> that the hashsize (measured in number of entries) would be a power of two,
> which the input hash function for that table requires.  The root cause of
> the problem is that two values need to be computed (one, the allocation
> order of the storage requries, as passed to __get_free_pages, and two the
> number of entries for the hash table).  Both need to be ^2, but for
> different reasons, and the existing code is simply computing one order
> value, and using it as the basis for both, which is wrong (i.e. it assumes
> that ((1<<order)*PAGE_SIZE)/sizeof(bucket) is still ^2 when its not).
> 
> To fix this, we change the logic slightly.  We start by computing a goal
> allocation order (which is limited by the maximum size hash table we want
> to support.  Then we attempt to allocate that size table, decreasing the
> order until a successful allocation is made.  Then, with the resultant
> successful order we compute the number of buckets that hash table supports,
> which we then round down to the nearest power of two, giving us the number
> of entries the table actually supports.
> 
> I've tested this locally here, using non-debug and spinlock-debug kernels,
> and the number of entries in the hashtable consistently work out to be
> powers of two in all cases.
> 
> Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
> Reported-by: Dmitry Vyukov <dvyukov@google.com>

This needs some work:

In file included from include/linux/list.h:8:0,
  from include/linux/module.h:9, from net/sctp/protocol.c:44:
  net/sctp/protocol.c: In function ‘sctp_init’:
  include/linux/kernel.h:752:17: warning: comparison of distinct
  pointer types lacks a cast (void) (&_min1 == &_min2); \ ^
  net/sctp/protocol.c:1431:10: note: in expansion of macro ‘min’ order
  = min(order, max_entry_order); ^ In file included from
  include/linux/printk.h:6:0, from include/linux/kernel.h:13, from
  include/linux/list.h:8, from include/linux/module.h:9, from
  net/sctp/protocol.c:44:
include/linux/kern_levels.h:4:18: warning: format ‘%d’ expects argument of type ‘int’, but argument 3 has type ‘long unsigned int’ [-Wformat=]
 #define KERN_SOH "\001"  /* ASCII Start Of Header */
                  ^
include/linux/kern_levels.h:13:19: note: in expansion of macro ‘KERN_SOH’
 #define KERN_INFO KERN_SOH "6" /* informational */
                   ^
include/linux/printk.h:259:9: note: in expansion of macro ‘KERN_INFO’
  printk(KERN_INFO pr_fmt(fmt), ##__VA_ARGS__)
         ^
net/sctp/protocol.c:1484:2: note: in expansion of macro ‘pr_info’
  pr_info("Hash tables configured (bind %d/%d)\n", sctp_port_hashsize,
  ^