Re: [PATCH v3 3/3] mm/memory.c: Add memory read privilege before filling PTE entry

[maobibo <maobibo@loongson.cn>]

On 05/28/2020 04:55 AM, Hugh Dickins wrote:
> On Tue, 19 May 2020, maobibo wrote:
>> On 05/19/2020 04:57 AM, Andrew Morton wrote:
>>> On Mon, 18 May 2020 13:08:49 +0800 Bibo Mao <maobibo@loongson.cn> wrote:
>>>
>>>> On mips platform, hw PTE entry valid bit is set in pte_mkyoung
>>>> function, it is used to set physical page with readable privilege.
>>>
>>> pte_mkyoung() seems to be a strange place to set the pte's valid bit. 
>>> Why is it done there?  Can it be done within mips's mk_pte()?
>> On MIPS system hardware cannot set PAGE_ACCESS bit when accessing the page,
>> software sets PAGE_ACCESS software bit and PAGE_VALID hw bit together during page
>> fault stage.
>>
>> If mk_pte is called in page fault flow, it is ok to set both bits. If it is not 
>> called in page fault, PAGE_ACCESS is set however there is no actual memory accessing.
> 
> Sorry for joining in so late, but would you please explain that some more:
> preferably in the final commit message, if not here.
> 
> I still don't understand why this is not done in the same way as on other
> architectures - those that care (I just checked x86, powerpc, arm, arm64,
> but not all of them) make sure that all the bits they want are there in
> mm/mmap.c's protection_map[16], which then feeds into vma->vm_page_prot,
> and so into mk_pte() as Andrew indicated.
> 
> And I can see that arch/mips/mm/cache.c has a setup_protection_map()
> to do that: why does it not set the additional bits that you want?
> including the valid bit and the accessed (young) bit, as others do.
> Are you saying that there are circumstances in which it is wrong
> for mk_pte() to set the additional bits?
MIPS is actually strange here, _PAGE_ACCESSED is not set in protection_map.
I do not understand history of mips neither. 

On x86/aarch/powerpc system, _PAGE_ACCESSED bit is set in the beginning. How does
software track memory page accessing frequency?  Does software not care current
status about _PAGE_ACCESSED bit, just calles madvise_cold to clear this bit, and
then watches whether this bit is changed or not?

regards
bibo,mao
> 
> I'm afraid that generic mm developers will have no clue as to whether
> or not to add a pte_sw_mkyoung() after a mk_pte(); and generic source
> will be the cleaner if it turns out not to be needed (but thank you
> for making sure that it does nothing on the other architectures).
> 
> Hugh
>