Re: vmalloc fault

[Rik van Riel <riel@redhat.com>]

On 04/26/2013 06:17 AM, Simon Jeons wrote:
> Morning Rik, so early.
> On 04/26/2013 06:04 PM, Rik van Riel wrote:
>> On 04/26/2013 05:55 AM, Simon Jeons wrote:
>>> Hi all,
>>>
>>> 1. Why vmalloc fault need sync user process page table with kernel page
>>> table instead of using kernel page table directly?
>>
>> Each process has its own PGD, into which both kernel and user
>> PMDs (or PUDs) are mapped. It is possible the PGD is missing
>> some pointers, that need to be filled in at fault time.
>
> It seems that you miss my question. I mean why sync page table between
> user process and init_mm.pgd, can user process use init_mm.pgd page
> table directly when access vmalloc memory? ;-)

init_mm.pgd is never used as an actual mmu context after bootup.

>>
>>> 2. Why do_swap_page doesn't set present flag?
>>
>> It does.  Look at how vm_get_page_prot works.
>
> Got it.
>
>>
>>> 3. When enable DEBUG_PAGEALLOC(catch use-after-free bug), if user
>>> process alloc pages from zone_normal(which is direct mapping) when
>>> fallback, this page which allocated for user process will set present
>>> flag in related pte, correct? but why also set present flag for kernel
>>> direct mapping? Does kernel have any requirement to access it?
>>
>>
>


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