On 8/10/23 03:26, David Hildenbrand wrote: > On 09.08.23 21:21, Matthew Wilcox wrote: >> On Wed, Aug 09, 2023 at 08:07:43PM +0100, Ryan Roberts wrote: >>>> +++ b/mm/hugetlb.c >>>> @@ -1479,7 +1479,7 @@ static void __destroy_compound_gigantic_folio(struct folio *folio, >>>> struct page *p; >>>> atomic_set(&folio->_entire_mapcount, 0); >>>> - atomic_set(&folio->_nr_pages_mapped, 0); >>>> + atomic_set(&folio->_total_mapcount, 0); >>> >>> Just checking this is definitely what you intended? _total_mapcount is -1 when >>> it means "no pages mapped", so 0 means 1 page mapped? >> >> We're destroying the page here, so rather than setting the meaning of >> this, we're setting the contents of this memory to 0. >> >> >> Other thoughts that ran through my mind ... can we wrap? I don't think >> we can; we always increment total_mapcount by 1, no matter whether we're >> incrementing entire_mapcount or an individual page's mapcount, and we >> always call folio_get() first, so we can't increment total_mapcount >> past 2^32 because folio_get() will die first. We might be able to >> wrap past 2^31, but I don't think so. > > From my understanding, if we wrap the total mapcount, we already wrapped the refcount -- as you say, grabbing a reference ahead of time for each mapping is mandatory. Both are 31bit values. We could treat the total mapcount as an unsigned int, but that's rather future work. > > Also, even folio_mapcount() and total_mapcount() return an "int" as of now. > > But yes, I also thought about that. In the future we might want (at least) for bigger folios refcount+total_mapcount to be 64bit. Or we manage to decouple both and only have the total_mapcount be 64bit only. This means pvmw may need to check more than 2^32 pte entries. :). I hope we have other way to get bigger folio and keep mapcount not too big. Regards Yin, Fengwei >
