Re: [gentoo-user] re: NX (Execute Disable) protection cannot be enabled: non-PAE kernel! [dmesg]

[Alan McKinnon <alan.mckinnon@gmail.com>]

On 08/10/2013 19:04, Alexander Kapshuk wrote:
> On 10/07/2013 10:42 PM, Alan McKinnon wrote:
>> On 07/10/2013 21:28, Alexander Kapshuk wrote:
>>> Is the message below I should do something about?
>>>
>>> box0=; dmesg|grep -i PAE
>>> [    0.000000] Notice: NX (Execute Disable) protection cannot be
>>> enabled: non-PAE kernel!
>>>
>>>
>>> My CPU seems to have support for it.
>>> box0=; grep pae /proc/cpuinfo
>>> flags        : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca
>>> cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe nx lm
>>> constant_tsc arch_perfmon pebs bts aperfmperf pni dtes64 monitor ds_cpl
>>> est tm2 ssse3 cx16 xtpr pdcm lahf_lm dtherm
>>> flags        : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca
>>> cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe nx lm
>>> constant_tsc arch_perfmon pebs bts aperfmperf pni dtes64 monitor ds_cpl
>>> est tm2 ssse3 cx16 xtpr pdcm lahf_lm dtherm
>>>
>>> But not my kernel, as far as I can tell.
>>> box0=; uname -a
>>> Linux box0 3.10.7-gentoo-r1 #1 SMP Sat Oct 5 23:57:58 EEST 2013 i686
>>> Intel(R) Pentium(R) Dual CPU T3400 @ 2.16GHz GenuineIntel GNU/Linux
>>>
>>> box0=; grep -i pae .config
>>> box0=; echo $?
>>> 1
>>>
>>> Thanks.
>>>
>>>
>>
>> Symbol: ARM_LPAE [=ARM_LPAE]
>> Type  : unknown
>>
>> Symbol: X86_PAE [=n]
>> Type  : boolean
>> Prompt: PAE (Physical Address Extension) Support
>>   Location:
>> (1) -> Processor type and features
>>   Defined at arch/x86/Kconfig:1213
>>   Depends on: X86_32 [=n] && !HIGHMEM4G [=n]
>>   Selected by: HIGHMEM64G [=n] && <choice> && !M486 [=n]
>>
>>
> Thanks. I did a little research into the whole PAE business, as
> obviously my knowledge of the matter in question was limited.
> 
> If I understood it correctly, PAE is a method that allows 32-bit kernels
> access 4 or more Gb of RAM, which is normally the domain of 64-bit kernels.
> 
> Seeing I've got 3 Gb of RAM, I arrived at the conclusion I don't need
> PAE enabled, do I?
> 
> box0=; sed 1q /proc/meminfo
> MemTotal:        3107668 kB


That is correct, with 3G physica RAM, you will not benefit from using
PAE at all. I don't think it interferes with anything if you do have it,
I recall a time when RedHat shipped 32 bit kernels that were PAE-enabled.

Briefly, the way it works is that the kernel assigns blocks of memory to
different processes. So a single process can still only access 4G of
memory, but two different process don't anymore have to address the same
4G of memory like you must do without PAE. But you still don't get to
give your 32 bit database more than 4g of RAM




-- 
Alan McKinnon
alan.mckinnon@gmail.com