MIME-Version: 1.0
References: <20211123000102.4052105-1-almasrymina@google.com>
 <YaMBGQGNLqPd6D6f@casper.infradead.org> <CAHS8izM5as_AmN4bSmZd1P7aSXZ86VAfXgyooZivyf7-E5gZcQ@mail.gmail.com>
In-Reply-To: <CAHS8izM5as_AmN4bSmZd1P7aSXZ86VAfXgyooZivyf7-E5gZcQ@mail.gmail.com>
From:   Mina Almasry <almasrymina@google.com>
Date:   Tue, 4 Jan 2022 15:04:31 -0800
Message-ID: <CAHS8izNw87-L=rEwJF7_9WCaAcXLn2dUe68h_SbLErJoSUDzzg@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
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

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?