Re: [PATCH 0/2] don't use mapcount() to check large folio sharing

[Yin, Fengwei]

On 8/4/2023 4:46 AM, Yu Zhao wrote:
> On Wed, Aug 2, 2023 at 6:56 AM Yin, Fengwei <fengwei.yin@xxxxxxxxx> wrote:
>>
>>
>>
>> On 8/2/2023 8:49 PM, Ryan Roberts wrote:
>>> On 02/08/2023 13:42, Yin, Fengwei wrote:
>>>>
>>>>
>>>> On 8/2/2023 8:40 PM, Ryan Roberts wrote:
>>>>> On 02/08/2023 13:35, Yin, Fengwei wrote:
>>>>>>
>>>>>>
>>>>>> On 8/2/2023 6:27 PM, Ryan Roberts wrote:
>>>>>>> On 28/07/2023 17:13, Yin Fengwei wrote:
>>>>>>>> In madvise_cold_or_pageout_pte_range() and madvise_free_pte_range(),
>>>>>>>> folio_mapcount() is used to check whether the folio is shared. But it's
>>>>>>>> not correct as folio_mapcount() returns total mapcount of large folio.
>>>>>>>>
>>>>>>>> Use folio_estimated_sharers() here as the estimated number is enough.
>>>>>>>>
>>>>>>>> Yin Fengwei (2):
>>>>>>>>   madvise: don't use mapcount() against large folio for sharing check
>>>>>>>>   madvise: don't use mapcount() against large folio for sharing check
>>>>>>>>
>>>>>>>>  mm/huge_memory.c | 2 +-
>>>>>>>>  mm/madvise.c     | 6 +++---
>>>>>>>>  2 files changed, 4 insertions(+), 4 deletions(-)
>>>>>>>>
>>>>>>>
>>>>>>> As a set of fixes, I agree this is definitely an improvement, so:
>>>>>>>
>>>>>>> Reviewed-By: Ryan Roberts
>>>>>> Thanks.
>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> But I have a couple of comments around further improvements;
>>>>>>>
>>>>>>> Once we have the scheme that David is working on to be able to provide precise
>>>>>>> exclusive vs shared info, we will probably want to move to that. Although that
>>>>>>> scheme will need access to the mm_struct of a process known to be mapping the
>>>>>>> folio. We have that info, but its not passed to folio_estimated_sharers() so we
>>>>>>> can't just reimplement folio_estimated_sharers() - we will need to rework these
>>>>>>> call sites again.
>>>>>> Yes. This could be extra work. Maybe should delay till David's work is done.
>>>>>
>>>>> What you have is definitely an improvement over what was there before. And is
>>>>> probably the best we can do without David's scheme. So I wouldn't delay this.
>>>>> Just pointing out that we will be able to make it even better later on (if
>>>>> David's stuff goes in).
>>>> Yes. I agree that we should wait for David's work ready and do fix based on that.
>>>
>>> I was suggesting the opposite - not waiting. Then we can do separate improvement
>>> later.
>> Let's wait for David's work ready.
> 
> Waiting is fine as long as we don't miss the next merge window -- we
> don't want these two bugs to get into another release. Also I think we
> should cc stable, since as David mentioned, they have been causing
> selftest failures.

Stable was CCed. Andrew asked about the user-visible impact and I replied:
https://lore.kernel.org/linux-mm/24e7429c-14ed-d953-e652-eac178de76e3@xxxxxxxxx/

I was not aware that selftest is impact by the issue. And waiting for whether these
patches are necessary for stable.

But I don't want to promote this kind of change in other place as we will move to
David's solution.


Regards
Yin, Fengwei