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

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Date:   Sun, 28 Nov 2021 04:10:01 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Mina Almasry <almasrymina@...gle.com>
Cc:     Jonathan Corbet <corbet@....net>,
        David Hildenbrand <david@...hat.com>,
        "Paul E . McKenney" <paulmckrcu@...com>,
        Yu Zhao <yuzhao@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Xu <peterx@...hat.com>,
        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.

But what is userspace going to _do_ differently if the kernel hasn't
backed the memory with huge pages?