Date:   Mon, 22 Nov 2021 17:50:44 -0800 (PST)
From:   David Rientjes <rientjes@google.com>
To:     Mina Almasry <almasrymina@google.com>
cc:     Jonathan Corbet <corbet@lwn.net>,
        David Hildenbrand <david@redhat.com>,
        Matthew Wilcox <willy@infradead.org>,
        "Paul E . McKenney" <paulmckrcu@fb.com>,
        Yu Zhao <yuzhao@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Xu <peterx@redhat.com>,
        Ivan Teterevkov <ivan.teterevkov@nutanix.com>,
        Florian Schmidt <florian.schmidt@nutanix.com>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-mm@kvack.org, linux-doc@vger.kernel.org
Subject: Re: [PATCH v7] mm: Add PM_THP_MAPPED to /proc/pid/pagemap
In-Reply-To: <20211123000102.4052105-1-almasrymina@google.com>
Message-ID: <b34e16a-f520-ec7b-7811-6adc2e645a5@google.com>
References: <20211123000102.4052105-1-almasrymina@google.com>
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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>