From: Neil Horman <nhor...@tuxdriver.com> Date: Thu, 18 Feb 2016 16:10:57 -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 <nhor...@tuxdriver.com> > Reported-by: Dmitry Vyukov <dvyu...@google.com> Applied and queued up for -stable, thanks Neil.
