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

Ryan Roberts
2023-08-02T12:50:15Z

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.

>
>
> Regards
> Yin, Fengwei
>
>>
>>>
>>>>
>>>> Given the aspiration for most of the memory to be large folios going forwards,
>>>> wouldn't it be better to avoid splitting the large folio where the large folio
>>>> is mapped entirely within the range of the madvise operation? Sorry if this has
>>>> already been discussed and decided against - I didn't follow the RFC too
>>>> closely. Or perhaps you plan to do this as a follow up?
>>> Yes. We are on same page. RFC patchset did that. But there are some other opens
>>> on the RFC. So I tried to submit this part of change which is bug fix. The other
>>> thing left in RFC is optimization (avoid split large folio if we can).
>>>
>>>
>>> Regards
>>> Yin, Fengwei
>>>
>>>>
>>>> Thanks,
>>>> Ryan
>>>>
>>
>>