Re: [PATCH v2] MIPS64: Support of at least 48 bits of SEGBITS

[Joshua Kinard <kumba@gentoo.org>]

On 05/15/2015 18:39, Leonid Yegoshin wrote:
> On 05/15/2015 02:53 PM, Ralf Baechle wrote:
>> On Thu, May 14, 2015 at 06:34:43PM -0700, Leonid Yegoshin wrote:
>>
>> The order 1 allocation for the PGD are concerning me a little.  On a
>> system under even moderate memory pressure that might become a bit of
>> a reliability or performance issue.
>>
>> With 4kB pages we already need order 1 or even 2 allocations for the
>> allocation of the stack and some folks have reported that to be an issue
>> so we may have to start using the PUD for very large VA spaces.
>>
>>    Ralf
> 
> I don't think it is an issue here - people, who wants to exercise 256 TERABAIT
> of memory PER PROCESS may even doesn't note that they have PGD = 2 pages. It is
> definitely not for systems with 4GB physmemory.
> 
> I also recommend for low memory to look into CONFIG_COMPACTION, it may be a
> great help for them here, look into mm/vmscan.c, in_reclaim_compaction().
> 
> Besides that, I defined this feature for 16KB and 64KB pages only, not for 4KB.

There's something screwy with R14000 CPUs and 16kb/64kb pages.  I haven't
figured it out yet, but you get random SIGSEGV and SIGBUS errors running that
PAGE_SIZE.  I figure it's some unknown/undocumented bit that SGI added and
never disclosed anywhere.

--J