linux-kernel - Re: [RFC] nvfs: a filesystem for persistent memory

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Date:   Mon, 21 Sep 2020 12:19:07 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>
cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Eric Sandeen <esandeen@...hat.com>,
        Dave Chinner <dchinner@...hat.com>,
        "Kani, Toshi" <toshi.kani@....com>,
        "Norton, Scott J" <scott.norton@....com>,
        "Tadakamadla, Rajesh (DCIG/CDI/HPS Perf)" 
        <rajesh.tadakamadla@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>
Subject: Re: [RFC] nvfs: a filesystem for persistent memory



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)?

I've added mount options dax=never, dax=auto, dax=always, so that the user 
can override the automatic behavior.

Mikulas