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

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

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.

> 
> 
> 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?

> But we should definitely spell that out.
>