PAE broken on 7-STABLE

[Arnaud Lacombe]

Hi,

On Mon, Dec 5, 2011 at 10:02 PM, Alan Cox <alan.l.cox at gmail.com> wrote:
> On Mon, Dec 5, 2011 at 4:15 PM, Arnaud Lacombe <lacombar at gmail.com> wrote:
>>
>> Hi,
>>
>> A FreeBSD 7-STABLE miserably crashes on the following:
>>
>> Fatal trap 12: page fault while in kernel mode
>> cpuid = 0; apic id = 00
>> fault virtual address = 0xbfef0000
>> fault code            = supervisor read, page not present
>> instruction pointer   = 0x20:0xc05fd1c2
>> stack pointer         = 0x28:0xc0af6c7c
>> frame pointer         = 0x28:0xc0af6cc0
>> code segment          = base 0x0, limit 0xfffff, type 0x1b
>>                      = DPL 0, pres 1, def32 1, gran 1
>> processor eflags      = interrupt enabled, resume, IOPL = 0
>> current process       = 0 ()
>> trap number           = 12
>> panic: page fault
>> cpuid = 0
>> KDB: stack
>> backtrace:
>> db_trace_self_wrapper(c0662728,0,c062b78b,c0af6b28,0,...) at
>> db_trace_self_wrapper+0x26panic(c062b78b,c06639cc,c06c1de4,1,1,...) at
>> panic+0x106trap_fatal(c0c74388,c065b897,c064d922,10,c0c74000,...) at
>> trap_fatal+0x270
>> trap_pfault(c06d4e40,c0c74380,c0af6c40,3,c06c1bc0,...) at
>> trap_pfault+0x2aa
>> trap(c0af6c3c) at trap+0x36ecalltrap() at calltrap+0x6
>> --- trap 0xc, eip = 0xc05fd1c2, esp = 0xc0af6c7c, ebp = 0xc0af6cc0 ---
>> pmap_map(c0af6d68,3f6ba000,6,3fef8000,6,...) at pmap_map+0x72
>> vm_page_startup(c0d3e000,a,c0af6d88,c03f8f26,0,...) at
>> vm_page_startup+0x35a
>> vm_mem_init(0,af0000,af0020,af0000,0,...) at vm_mem_init+0x18
>> mi_startup() at mi_startup+0x56begin() at begin+0x2c
>>
>> on a machine with 24GB of RAM, while PAE is meant to support up to 64GB.
>>
>>  - Arnaud
>>
>> ps: this is just a report, I'm not really expecting anything, any
>> longer, from the FreebSD "community".
>>
>
> At this early stage in the boot process, the page table pages for the kernel
> address space must be statically allocated.  When PAE was still actively
> used, it was unusual to find machines that had more than about 16GB of RAM.
> So, the static allocation of page table pages was set accordingly.  For
> larger machines, it is necessary to increase NKPT.  The following comment
> appears in i386/include/pmap.h:
>
> /* Initial number of kernel page tables. */
> #ifndef NKPT
> #ifdef PAE
> /* 152 page tables needed to map 16G (76B "struct vm_page", 2M page tables).
> */
> #define NKPT            240
> #else
> /* 18 page tables needed to map 4G (72B "struct vm_page", 4M page tables).
> */
> #define NKPT            30
> #endif
> #endif
>
> That said, a machine with 24GB of RAM is likely not going to be usable for
> many workloads unless you also increase the size of the kernel virtual
> address space (and thereby reduce the size of the user virtual address
> space).
>
this might be a silly question to ask, but why FreeBSD's VM (or at
least seems to) performs so badly compared to Linux' VM ?

Thanks,
 - Arnaud