linux-kernel - Re: [PATCH v7] mm: Add PM_THP

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Date:   Tue, 23 Nov 2021 09:10:40 +0800
From:   Peter Xu <peterx@...hat.com>
To:     Mina Almasry <almasrymina@...gle.com>
Cc:     Jonathan Corbet <corbet@....net>,
        David Hildenbrand <david@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        "Paul E . McKenney" <paulmckrcu@...com>,
        Yu Zhao <yuzhao@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ivan Teterevkov <ivan.teterevkov@...anix.com>,
        Florian Schmidt <florian.schmidt@...anix.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH v7] mm: Add PM_THP_MAPPED to /proc/pid/pagemap

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.
> 
> 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@...gle.com>

Acked-by: Peter Xu <peterx@...hat.com>

-- 
Peter Xu