Re: [PATCH v5] mm: Add PM_HUGE_THP_MAPPING to /proc/pid/pagemap

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 10 Nov 2021 14:11:20 -0800 Mina Almasry <almasrymina@google.com> wrote:

> Add PM_HUGE_THP 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.
> 
> PM_HUGE_THP_MAPPING bit is set if the virt address is mapped at the PMD
> level and the underlying page is a transparent huge page.
> 
> A few options were considered:
> 1. Add /proc/pid/pageflags that exports the same info as
>    /proc/kpageflags.  This is not appropriate because many kpageflags are
>    inappropriate to expose to userspace processes.
> 2. Simply get this info from the existing /proc/pid/smaps interface.
>    There are a couple of issues with that:
>    1. /proc/pid/smaps output is human readable and unfriendly to
>       programatically parse.
>    2. /proc/pid/smaps is slow.  The cost of reading /proc/pid/smaps into
>       userspace buffers is about ~800us per call, and this doesn't
>       include parsing the output to get the information you need. The
>       cost of querying 1 virt address in /proc/pid/pagemaps however is
>       around 5-7us.
> 
> Tested manually by adding logging into transhuge-stress, and by
> allocating THP and querying the PM_HUGE_THP_MAPPING flag at those
> virtual addresses.
> 
> --- a/tools/testing/selftests/vm/transhuge-stress.c
> +++ b/tools/testing/selftests/vm/transhuge-stress.c
> @@ -16,6 +16,12 @@
>  #include <string.h>
>  #include <sys/mman.h>
> 
> +/*
> + * We can use /proc/pid/pagemap to detect whether the kernel was able to find
> + * hugepages or no. This can be very noisy, so is disabled by default.
> + */
> +#define NO_DETECT_HUGEPAGES
> +
>
> ...
>
> +#ifndef NO_DETECT_HUGEPAGES
> +		if (!PAGEMAP_THP(ent[0]))
> +			fprintf(stderr, "WARNING: detected non THP page\n");
> +#endif

This looks like a developer thing.  Is there any point in leaving it in
the mainline code?