Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Mon, 16 Aug 2021 15:27:55 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     David Hildenbrand <david@redhat.com>
Cc:     Khalid Aziz <khalid.aziz@oracle.com>,
        "Longpeng (Mike, Cloud Infrastructure Service Product Dept.)" 
        <longpeng2@huawei.com>, Steven Sistare <steven.sistare@oracle.com>,
        Anthony Yznaga <anthony.yznaga@oracle.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "Gonglei (Arei)" <arei.gonglei@huawei.com>
Subject: Re: [RFC PATCH 0/5] madvise MADV_DOEXEC
Message-ID: <YRp169xvwB3j0rpD@casper.infradead.org>
References: <YOubKmDwxMIvdAed@casper.infradead.org>
 <a94973ab83ce48bd85c91397f82d7915@huawei.com>
 <55720e1b39cff0a0f882d8610e7906dc80ea0a01.camel@oracle.com>
 <db2b7337-4c6b-4e4b-71d3-dc4940353498@redhat.com>
 <YRpVHnr55LpQQvTb@casper.infradead.org>
 <ca2d4ea4-e875-475a-6094-1ac58bc0b544@redhat.com>
 <YRpeHnP7QDNJRA8Y@casper.infradead.org>
 <88884f55-4991-11a9-d330-5d1ed9d5e688@redhat.com>
 <YRpo4EAJSkY7hI7Q@casper.infradead.org>
 <40bad572-501d-e4cf-80e3-9a8daa98dc7e@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <40bad572-501d-e4cf-80e3-9a8daa98dc7e@redhat.com>
Precedence: bulk

On Mon, Aug 16, 2021 at 04:10:28PM +0200, David Hildenbrand wrote:
> > > > Until recently, the CPUs only having 4 1GB TLB entries.  I'm sure we
> > > > still have customers using that generation of CPUs.  2MB pages perform
> > > > better than 1GB pages on the previous generation of hardware, and I
> > > > haven't seen numbers for the next generation yet.
> > > 
> > > I read that somewhere else before, yet we have heavy 1 GiB page users,
> > > especially in the context of VMs and DPDK.
> > 
> > I wonder if those users actually benchmarked.  Or whether the memory
> > savings worked out so well for them that the loss of TLB performance
> > didn't matter.
> 
> These applications are extremely performance sensitive (i.e., RT workloads),

"real time does not mean real fast".  it means predictable latency.

> > > I will rephrase my previous statement "hugetlbfs just doesn't raise these
> > > problems because we are special casing it all over the place already". For
> > > example, not allowing to swap such pages. Disallowing MADV_DONTNEED. Special
> > > hugetlbfs locking.
> > 
> > Sure, that's why I want to drag this feature out of "oh this is a
> > hugetlb special case" and into "this is something Linux supports".
> 
> I would have understood the move to optimize SHMEM internally - similar to
> how we seem to optimize hugetlbfs SHMEM right now internally. (although
> sharing page tables for shmem can still be quite tricky)
> 
> I did not follow why we have to play games with MAP_PRIVATE, and having
> private anonymous pages shared between processes that don't COW, introducing
> new syscalls etc.

It's not about SHMEM, it's about file-backed pages on regular
filesystems.  I don't want to have XFS, ext4 and btrfs all with their
own implementations of ARCH_WANT_HUGE_PMD_SHARE.