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

[David Hildenbrand <david@redhat.com>]

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.

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

-- 
Cheers,

David / dhildenb