On Wed, Jan 25, 2023 at 10:16:58AM -0800, Mike Kravetz wrote: > On 01/25/23 11:46, Peter Xu wrote: > > On Tue, Jan 24, 2023 at 03:35:38PM -0800, Mike Kravetz wrote: > > > On 01/24/23 18:00, Peter Xu wrote: > > > > On Tue, Jan 24, 2023 at 12:56:24PM -0800, Mike Kravetz wrote: > > > > Besides, I'm also curious on the planned fix too regarding the two issues > > > > mentioned. > > > > > > My planned 'fix' is to simply check for shared a PMD > > > (page_count(virt_to_page(pte))) to determine if page with mapcount == 1 > > > is shared. > > > > I think having the current pte* won't easily work, we'll need to walk all > > the pgtable that mapped this page. > > > > To be explicit, one page can be mapped at pgtable1 which is shared by proc1 > > & proc2, and it can also be mapped at pgtable2 which is shared by proc3 & > > proc4. Then (assuming pte1* points to pgtable1): > > > > page_count(virt_to_page(pte1)) + page_mapcount(page) > > > > Won't be the right mapcount we're looking for. > > That assumes we want an accurate mapcount. In the two code segments I pointed > out, we only need to know if more than one process maps the page. We can > get that with 'page_count(virt_to_page(pte)) + page_mapcount(page)'. I see. Yes it sounds working. We can check mapcount==1 first and only go for the pgtable if mapcount>1, then we also know that's the only pmd it got mapped at. > > For now (and stable releases), I propose just fixing the improper behavior. > If we really need an accurate mapcount in the shared PMD case (and I am not > 100% sure we do), then we can add code to maintain or compute that. > > Part of my reason for sending this email was to determine if there may > be some places where we really do need an accurate mapcount in the shared > PMD case. I have not found any, but could have easily missed something. Nothing I noticed either. Thanks, -- Peter Xu
