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

[David Rientjes <rientjes@google.com>]

On Mon, 22 Nov 2021, 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.
> 
> PM_THP_MAPPED 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 because it must read the whole memory range
>       rather than a small range we care about.  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_THP_MAPPED flag at those
> virtual addresses.
> 
> Signed-off-by: Mina Almasry <almasrymina@google.com>

Acked-by: David Rientjes <rientjes@google.com>