On Tue, Nov 23, 2021 at 01:10:37PM -0800, Mina Almasry wrote:

On Tue, Nov 23, 2021 at 12:51 PM Matthew Wilcox [off-list ref] wrote:

quoted

On Mon, Nov 22, 2021 at 04:01:02PM -0800, Mina Almasry wrote:

quoted

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.