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From:   Dan Williams <dan.j.williams@intel.com>
Date:   Mon, 21 Sep 2020 09:29:39 -0700
Message-ID: <CAPcyv4iys4xDnDyuzo4yj1MvOWXkQ_NgPfupB=btZt_kzi29Ug@mail.gmail.com>
Subject: Re: [RFC] nvfs: a filesystem for persistent memory
To:     Mikulas Patocka <mpatocka@redhat.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vishal Verma <vishal.l.verma@intel.com>,
        Dave Jiang <dave.jiang@intel.com>,
        Ira Weiny <ira.weiny@intel.com>,
        Matthew Wilcox <willy@infradead.org>, Jan Kara <jack@suse.cz>,
        Eric Sandeen <esandeen@redhat.com>,
        Dave Chinner <dchinner@redhat.com>,
        "Kani, Toshi" <toshi.kani@hpe.com>,
        "Norton, Scott J" <scott.norton@hpe.com>,
        "Tadakamadla, Rajesh (DCIG/CDI/HPS Perf)" 
        <rajesh.tadakamadla@hpe.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        linux-nvdimm <linux-nvdimm@lists.01.org>
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On Mon, Sep 21, 2020 at 9:19 AM Mikulas Patocka <mpatocka@redhat.com> wrote:
>
>
>
> On Tue, 15 Sep 2020, Dan Williams wrote:
>
> > > TODO:
> > >
> > > - programs run approximately 4% slower when running from Optane-based
> > > persistent memory. Therefore, programs and libraries should use page cache
> > > and not DAX mapping.
> >
> > This needs to be based on platform firmware data f(ACPI HMAT) for the
> > relative performance of a PMEM range vs DRAM. For example, this
> > tradeoff should not exist with battery backed DRAM, or virtio-pmem.
>
> Hi
>
> I have implemented this functionality - if we mmap a file with
> (vma->vm_flags & VM_DENYWRITE), then it is assumed that this is executable
> file mapping - the flag S_DAX on the inode is cleared on and the inode
> will use normal page cache.
>
> Is there some way how to test if we are using Optane-based module (where
> this optimization should be applied) or battery backed DRAM (where it
> should not)?

No, there's no direct reliable type information. Instead the firmware
on ACPI platforms provides the HMAT table which provides performance
details of system-memory ranges.