Re: Problems with VM_MIXEDMAP removal from /proc/<pid>/smaps

[Dan Williams <dan.j.williams@intel.com>]

On Tue, Oct 2, 2018 at 8:32 AM Jan Kara <jack@suse.cz> wrote:
>
> On Tue 02-10-18 07:52:06, Christoph Hellwig wrote:
> > On Tue, Oct 02, 2018 at 04:44:13PM +0200, Johannes Thumshirn wrote:
> > > On Tue, Oct 02, 2018 at 07:37:13AM -0700, Christoph Hellwig wrote:
> > > > No, it should not.  DAX is an implementation detail thay may change
> > > > or go away at any time.
> > >
> > > Well we had an issue with an application checking for dax, this is how
> > > we landed here in the first place.
> >
> > So what exacty is that "DAX" they are querying about (and no, I'm not
> > joking, nor being philosophical).
>
> I believe the application we are speaking about is mostly concerned about
> the memory overhead of the page cache. Think of a machine that has ~ 1TB of
> DRAM, the database running on it is about that size as well and they want
> database state stored somewhere persistently - which they may want to do by
> modifying mmaped database files if they do small updates... So they really
> want to be able to use close to all DRAM for the DB and not leave slack
> space for the kernel page cache to cache 1TB of database files.

VM_MIXEDMAP was never a reliable indication of DAX because it could be
set for random other device-drivers that use vm_insert_mixed(). The
MAP_SYNC flag positively indicates that page cache is disabled for a
given mapping, although whether that property is due to "dax" or some
other kernel mechanics is purely an internal detail.

I'm not opposed to faking out VM_MIXEDMAP if this broken check has
made it into production, but again, it's unreliable.