Re: Problems with VM_MIXEDMAP removal from /proc//smaps

Dan Williams
2018-10-03T15:13:49Z

On Wed, Oct 3, 2018 at 8:07 AM Jan Kara <jack@suse.cz> wrote:
>
> On Wed 03-10-18 07:38:50, Dan Williams wrote:
> > On Wed, Oct 3, 2018 at 5:51 AM Jan Kara <jack@suse.cz> wrote:
> > >
> > > On Tue 02-10-18 13:18:54, Dan Williams wrote:
> > > > 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.
> > >
> > > So luckily this particular application wasn't widely deployed yet so we
> > > will likely get away with the vendor asking customers to update to a
> > > version not looking into smaps and parsing /proc/mounts instead.
> > >
> > > But I don't find parsing /proc/mounts that beautiful either and I'd prefer
> > > if we had a better interface for applications to query whether they can
> > > avoid page cache for mmaps or not.
> >
> > Yeah, the mount flag is not a good indicator either. I think we need
> > to follow through on the per-inode property of DAX. Darrick and I
> > discussed just allowing the property to be inherited from the parent
> > directory at file creation time. That avoids the dynamic set-up /
> > teardown races that seem intractable at this point.
> >
> > What's wrong with MAP_SYNC as a page-cache detector in the meantime?
>
> So IMHO checking for MAP_SYNC is about as reliable as checking for 'dax'
> mount option. It works now but nobody promises it will reliably detect DAX in
> future - e.g. there's nothing that prevents MAP_SYNC to work for mappings
> using pagecache if we find a sensible usecase for that.

Fair enough.

> WRT per-inode DAX property, AFAIU that inode flag is just going to be
> advisory thing - i.e., use DAX if possible. If you mount a filesystem with
> these inode flags set in a configuration which does not allow DAX to be
> used, you will still be able to access such inodes but the access will use
> page cache instead. And querying these flags should better show real
> on-disk status and not just whether DAX is used as that would result in an
> even bigger mess. So this feature seems to be somewhat orthogonal to the
> API I'm looking for.

True, I imagine once we have that flag we will be able to distinguish
the "saved" property and the "effective / live" property of DAX...
Also it's really not DAX that applications care about as much as "is
there page-cache indirection / overhead for this mapping?". That seems
to be a narrower guarantee that we can make than what "DAX" might
imply.