On Sat 18-04-20 11:12:07, chenqiwu wrote: > On Fri, Apr 17, 2020 at 05:23:31PM +0200, Michal Hocko wrote: > > On Fri 17-04-20 22:26:18, chenqiwu wrote: > > > On Fri, Apr 17, 2020 at 01:43:12PM +0200, Michal Hocko wrote: > > > > On Fri 17-04-20 18:31:54, qiwuchen55@xxxxxxxxx wrote: > > > > > From: chenqiwu <chenqiwu@xxxxxxxxxx> > > > > > > > > > > If we get a special mapping page like device mapping page or zero page > > > > > when copy_one_pte, it's necessary add the page refcount count. > > > > > > > > From the changelog it is not clear what is the actual problem and how > > > > the patch address it. Please be more verbose. > > > > > > > I don't find any actual problem, but I think there should be addressed > > > to update the page refcount for special mappings include devmap and zero > > > page instead of doing nothing else, since we copy the pte from one task > > > to the other. > > > > As Matthew pointed out, zero pages are special. Just check how > > vm_normal_page returns NULL (the same is the case for pte_devmap). This > > means, among other things that zap_pte_range which is called during > > munmap will only clear the pte but it doesn't operate on those pages so > > there is no put_page for your get_page here. > > > > I do realize that this might be a subtle details that might be > > confusing. On the other hand trying to formulate the specific problem > > and add an explanation of the fix in the changelog could have revealed > > this. It is really trivial to generate mappings backed by zero pages and > > if the reference count was not handled properly then it would blow up > > pretty quickly. > > > I agree. But I can't see where to put the normal page refcount back > in zap_pte_range(), is there an imbalnace between copy_one_pte() and > zap_pte_range()? No, there is no imbalance. The reference counter handling is very well hidden. Each page gets tracked by __tlb_remove_page and later handled tlb_flush_mmu. That all is done to optimize tlb flushing which is necessary when pages are freed. -- Michal Hocko SUSE Labs
