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

[Ryan Roberts <ryan.roberts@arm.com>]

On 02/08/2023 12:36, David Hildenbrand wrote:
> On 02.08.23 13:20, Ryan Roberts wrote:
>> On 02/08/2023 11:48, David Hildenbrand wrote:
>>> On 02.08.23 12:27, 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
>>>>
>>>>
>>>> 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
>>>
>>> There are probably ways to work around lack of mm_struct, but it would not be
>>> completely for free. But passing the mm_struct should probably be an easy
>>> refactoring.
>>>
>>>> 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.
>>>
>>> We should probably just have a
>>>
>>> folio_maybe_mapped_shared()
>>>
>>> with proper documentation. Nobody should care about the exact number.
>>>
>>>
>>> If my scheme for anon pages makes it in, that would be precise for anon pages
>>> and we could document that. Once we can handle pagecache pages as well to get a
>>> precise answer, we could change to folio_mapped_shared() and adjust the
>>> documentation.
>>
>> Makes sense to me. I'm assuming your change would allow us to get rid of
>> PG_anon_exclusive too? In which case we would also want a precise API
>> specifically for anon folios for the CoW case, without waiting for pagecache
>> page support.
> 
> Not necessarily and I'm currently not planning that
> 
> On the COW path, I'm planning on using it only when PG_anon_exclusive is clear
> for a compound page, combined with a check that there are no other page
> references besides from mappings: all mappings from me and #refs == #mappings ->
> reuse (set PG_anon_exclusive). That keeps the default (no fork) as fast as
> possible and simple.
> 
>>>
>>> I just saw
>>>
>>> https://lkml.kernel.org/r/20230802095346.87449-1-wangkefeng.wang@huawei.com
>>>
>>> that converts a lot of code to folio_estimated_sharers().
>>>
>>>
>>> That patchset, for example, also does
>>>
>>> total_mapcount(page) > 1 -> folio_estimated_sharers(folio) > 1
>>>
>>> I'm not 100% sure what to think about that at this point. We eventually add
>>> false negatives (actually shared but we fail to detect it) all over the place,
>>> instead of having false positives (actually exclusive, but we fail to detect
>>> it).
>>>
>>> And that patch set doesn't even spell that out.
>>>
>>>
>>> Maybe it's as good as we will get, especially if my scheme doesn't make it in.
>>
>> I've been working on the assumption that your scheme is plan A, and I'm waiting
>> for it to unblock forward progress on large anon folios. Is this the right
>> approach, or do you think your scheme is sufficiently riskly and/or far out that
>> I should aim not to depend on it?
> 
> It is plan A. IMHO, it does not feel too risky and/or far out at this point --
> and the implementation should not end up too complicated. But as always, I
> cannot promise anything before it's been implemented and discussed upstream.

OK, good we are on the same folio... (stolen from Hugh; if a joke is worth
telling once, its worth telling 1000 times ;-)

> 
> Hopefully, we know more soon. I'll get at implementing it fairly soon.
>