Re: [RFC] nvfs: a filesystem for persistent memory

[Ira Weiny]

On Mon, Sep 21, 2020 at 12:19:07PM -0400, Mikulas Patocka 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)?
> 
> I've added mount options dax=never, dax=auto, dax=always, so that the user 
                                      ^^^^^^^^
				      dax=inode?

'inode' is the option used by ext4/xfs.

Ira

> can override the automatic behavior.
> 
> Mikulas
>