On Tue, Nov 23, 2021 at 01:10:37PM -0800, Mina Almasry wrote: > On Tue, Nov 23, 2021 at 12:51 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > > > On Mon, Nov 22, 2021 at 04:01:02PM -0800, Mina Almasry wrote: > > > Add PM_THP_MAPPED MAPPING to allow userspace to detect whether a given virt > > > address is currently mapped by a transparent huge page or not. Example > > > use case is a process requesting THPs from the kernel (via a huge tmpfs > > > mount for example), for a performance critical region of memory. The > > > userspace may want to query whether the kernel is actually backing this > > > memory by hugepages or not. > > > > So you want this bit to be clear if the memory is backed by a hugetlb > > page? > > > > Yes I believe so. I do not see value in telling the userspace that the > virt address is backed by a hugetlb page, since if the memory is > mapped by MAP_HUGETLB or is backed by a hugetlb file then the memory > is backed by hugetlb pages and there is no vagueness from the kernel > here. > > Additionally hugetlb interfaces are more size based rather than PMD or > not. arm64 for example supports 64K, 2MB, 32MB and 1G 'huge' pages and > it's an implementation detail that those sizes are mapped CONTIG PTE, > PMD, CONITG PMD, and PUD respectively, and the specific mapping > mechanism is typically not exposed to the userspace and might not be > stable. Assuming pagemap_hugetlb_range() == PMD_MAPPED would not > technically be correct. What I've been trying to communicate over the N reviews of this patch series is that *the same thing is about to happen to THPs*. Only more so. THPs are going to be of arbitrary power-of-two size, not necessarily sizes supported by the hardware. That means that we need to be extremely precise about what we mean by "is this a THP?" Do we just mean "This is a compound page?" Do we mean "this is mapped by a PMD?" Or do we mean something else? And I feel like I haven't been able to get that information out of you.
