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From:   Mina Almasry <almasrymina@google.com>
Date:   Mon, 13 Dec 2021 16:22:19 -0800
Message-ID: <CAHS8izM5as_AmN4bSmZd1P7aSXZ86VAfXgyooZivyf7-E5gZcQ@mail.gmail.com>
Subject: Re: [PATCH v7] mm: Add PM_THP_MAPPED to /proc/pid/pagemap
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Jonathan Corbet <corbet@lwn.net>,
        David Hildenbrand <david@redhat.com>,
        "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
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On Sat, Nov 27, 2021 at 8:10 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> 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?

Sorry for the late reply here.

My plan is to expose this information as metrics right now and:
1. Understand the kind of hugepage backing we're actually getting if any.
2. If there are drops in hugepage backing we can investigate the
cause, whether it's due to normal memory fragmentation or some
bug/issue.
3. Schedule machines for reboots to defragment the memory if the
hugepage backing is too low.
4. Possibly motivate future work to improve hugepage backing if our
numbers are too low.