Re: [PATCH v7] mm: Add PM_THP_MAPPED to /proc/pid/pagemap

[Matthew Wilcox]

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?

>  		if (page && page_mapcount(page) == 1)
>  			flags |= PM_MMAP_EXCLUSIVE;
> +		if (page && is_transparent_hugepage(page))
> +			flags |= PM_THP_MAPPED;

because honestly i'd expect it to be more useful to mean "This memory
is mapped by a PMD entry" and then the code would look like:

		if (page)
			flags |= PM_PMD_MAPPED;

(and put a corresponding change in pagemap_hugetlb_range)