Re: [PATCH mm-unstable v1] mm: add a total mapcount for large folios

[Ryan Roberts]

On 10/08/2023 09:59, David Hildenbrand wrote:
> On 09.08.23 23:23, Peter Xu wrote:
>> Hi, David,
>>
>> Some pure questions below..
> 
> Hi Peter,
> 
> thanks for having a look!
> 
> [...]
> 
>>> With sub-PMD THP becoming more important and things looking promising
>>> that we will soon get support for such anon THP, we want to avoid looping
>>> over all pages of a folio just to calculate the total mapcount. Further,
>>> we may soon want to use the total mapcount in other context more
>>> frequently, so prepare for reading it efficiently and atomically.
>>
>> Any (perhaps existing) discussion on reduced loops vs added atomic
>> field/ops?
> 
> So far it's not been raised as a concern, so no existing discussion.
> 
> For order-0 pages the behavior is unchanged.
> 
> For PMD-mapped THP and hugetlb it's most certainly noise compared to the other
> activities when (un)mapping these large pages.
> 
> For PTE-mapped THP, it might be a bit bigger noise, although I doubt it is
> really significant (judging from my experience on managing PageAnonExclusive
> using set_bit/test_bit/clear_bit when (un)mapping anon pages).
> 
> As folio_add_file_rmap_range() indicates, for PTE-mapped THPs we should be
> batching where possible (and Ryan is working on some more rmap batching). 

Yes, I've just posted [1] which batches the rmap removal. That would allow you
to convert the per-page atomic_dec() into a (usually) single per-large-folio
atomic_sub().

[1] https://lore.kernel.org/linux-mm/20230810103332.3062143-1-ryan.roberts@xxxxxxx/

> There,
> managing the subpage mapcount dominates all other overhead significantly.
> 
>>
>> I had a feeling that there's some discussion behind the proposal of this
>> patch, if that's the case it'll be great to attach the link in the commit
>> log.
> 
> There were (mostly offline) discussions on how to sort out some other issues
> that PTE-mapped THP are facing, and how to eventually get rid of the subpage
> mapcounts (once consumer being _nr_pages_mapped as spelled out in the patch
> description). Having a proper total mapcount available was discussed as one
> building block.
> 
> I don't think I have anything of value to link that would make sense for the
> patch as is, as this patch is mostly independent from all that.
>