Date:   Wed, 5 Jan 2022 04:39:43 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Mina Almasry <almasrymina@google.com>
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
Subject: Re: [PATCH v7] mm: Add PM_THP_MAPPED to /proc/pid/pagemap
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On Tue, Jan 04, 2022 at 03:04:31PM -0800, Mina Almasry wrote:
> On Mon, Dec 13, 2021 at 4:22 PM Mina Almasry <almasrymina@google.com> wrote:
> >
> > 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.
> 
> Friendly ping on this. It has been reviewed by a few folks and after
> Matthew had questions about the use case which I've answered in the
> email above. Matthew, are you opposed to this patch?

I'm not convinced you need more than the existing stats
(THP_FAULT_FALLBACK) for the information you claim to want.