Re: [PATCH 1/1] mm/madvise: enhance lazyfreeing with mTHP in madvise_free

[Yin Fengwei <fengwei.yin@intel.com>]

On 2/27/24 14:40, Barry Song wrote:
> On Tue, Feb 27, 2024 at 7:14 PM Yin Fengwei <fengwei.yin@intel.com> wrote:
>>
>>
>>
>> On 2/27/24 10:17, Barry Song wrote:
>>>> Like if we hit folio which is partially mapped to the range, don't split it but
>>>> just unmap the mapping part from the range. Let page reclaim decide whether
>>>> split the large folio or not (If it's not mapped to any other range,it will be
>>>> freed as whole large folio. If part of it still mapped to other range,page reclaim
>>>> can decide whether to split it or ignore it for current reclaim cycle).
>>> Yes, we can. but we still have to play the ptes check game to avoid adding
>>> folios multiple times to reclaim the list.
>>>
>>> I don't see too much difference between splitting in madvise and splitting
>>> in vmscan.  as our real purpose is avoiding splitting entirely mapped
>>> large folios. for partial mapped large folios, if we split in madvise, then
>>> we don't need to play the game of skipping folios while iterating PTEs.
>>> if we don't split in madvise, we have to make sure the large folio is only
>>> added in reclaimed list one time by checking if PTEs belong to the
>>> previous added folio.
>>
>> If the partial mapped large folio is unmapped from the range, the related PTE
>> become none. How could the folio be added to reclaimed list multiple times?
> 
> in case we have 16 PTEs in a large folio.
> PTE0 present
> PTE1 present
> PTE2 present
> PTE3  none
> PTE4 present
> PTE5 none
> PTE6 present
> ....
> the current code is scanning PTE one by one.
> while scanning PTE0, we have added the folio. then PTE1, PTE2, PTE4, PTE6...
No. Before detect the folio is fully mapped to the range, we can't add folio
to reclaim list because the partial mapped folio shouldn't be added. We can
only scan PTE15 and know it's fully mapped.

So, when scanning PTE0, we will not add folio. Then when hit PTE3, we know
this is a partial mapped large folio. We will unmap it. Then all 16 PTEs
become none.

If the large folio is fully mapped, the folio will be added to reclaim list
after scan PTE15 and know it's fully mapped.

Regards
Yin, Fengwei

> 
> there are all kinds of possibilities for unmapping.
> 
> so what we can do is recording we have added the folio while scanning PTE0,
> then skipping this folios for all other PTEs.
> 
> otherwise, we can split it while scanning PTE0, then we will meet
> different folios
> afterwards.
> 
>>
>>
>> Regards
>> Yin, Fengwei
> 
> Thanks
> Barry