Re: DAX mapping detection (was: Re: [PATCH] Fix region lost in /proc/self/smaps)

[Darrick J. Wong]

On Mon, Sep 12, 2016 at 11:40:35AM +1000, Dave Chinner wrote:
> On Thu, Sep 08, 2016 at 04:56:36PM -0600, Ross Zwisler wrote:
> > On Wed, Sep 07, 2016 at 09:32:36PM -0700, Dan Williams wrote:
> > > My understanding is that it is looking for the VM_MIXEDMAP flag which
> > > is already ambiguous for determining if DAX is enabled even if this
> > > dynamic listing issue is fixed.  XFS has arranged for DAX to be a
> > > per-inode capability and has an XFS-specific inode flag.  We can make
> > > that a common inode flag, but it seems we should have a way to
> > > interrogate the mapping itself in the case where the inode is unknown
> > > or unavailable.  I'm thinking extensions to mincore to have flags for
> > > DAX and possibly whether the page is part of a pte, pmd, or pud
> > > mapping.  Just floating that idea before starting to look into the
> > > implementation, comments or other ideas welcome...
> > 
> > I think this goes back to our previous discussion about support for the PMEM
> > programming model.  Really I think what NVML needs isn't a way to tell if it
> > is getting a DAX mapping, but whether it is getting a DAX mapping on a
> > filesystem that fully supports the PMEM programming model.  This of course is
> > defined to be a filesystem where it can do all of its flushes from userspace
> > safely and never call fsync/msync, and that allocations that happen in page
> > faults will be synchronized to media before the page fault completes.
> > 
> > IIUC this is what NVML needs - a way to decide "do I use fsync/msync for
> > everything or can I rely fully on flushes from userspace?" 
> 
> "need fsync/msync" is a dynamic state of an inode, not a static
> property. i.e. users can do things that change an inode behind the
> back of a mapping, even if they are not aware that this might
> happen. As such, a filesystem can invalidate an existing mapping
> at any time and userspace won't notice because it will simply fault
> in a new mapping on the next access...
> 
> > For all existing implementations, I think the answer is "you need to use
> > fsync/msync" because we don't yet have proper support for the PMEM programming
> > model.
> 
> Yes, that is correct.
> 
> FWIW, I don't think it will ever be possible to support this ....
> wonderful "PMEM programming model" from any current or future kernel
> filesystem without a very specific set of restrictions on what can
> be done to a file.  e.g.
> 
> 	1. the file has to be fully allocated and zeroed before
> 	   use. Preallocation/zeroing via unwritten extents is not
> 	   allowed. Sparse files are not allowed. Shared extents are
> 	   not allowed.
> 	2. set the "PMEM_IMMUTABLE" inode flag - filesystem must
> 	   check the file is fully allocated before allowing it to
> 	   be set, and caller must have CAP_LINUX_IMMUTABLE.
> 	3. Inode metadata is now immutable, and file data can only
> 	   be accessed and/or modified via mmap().
> 	4. All non-mmap methods of inode data modification
> 	   will now fail with EPERM.
> 	5. all methods of inode metadata modification will now fail
> 	   with EPERM, timestamp udpdates will be ignored.
> 	6. PMEM_IMMUTABLE flag can only be removed if the file is
> 	   not currently mapped and caller has CAP_LINUX_IMMUTABLE.
> 
> A flag like this /should/ make it possible to avoid fsync/msync() on
> a file for existing filesystems, but it also means that such files
> have significant management issues (hence the need for
> CAP_LINUX_IMMUTABLE to cover it's use).

Hmmm... I started to ponder such a flag, but ran into some questions.
If it's PMEM_IMMUTABLE, does this mean that none of 1-6 apply if the
filesystem discovers it isn't on pmem?

I thought about just having a 'immutable metadata' flag where any
timestamp, xattr, or block mapping update just returns EPERM.  There
wouldn't be any checks as in (1); if you left a hole in the file prior
to setting the flag then you won't be filling it unless you clear the
flag.  OTOH if it merely made the metadata unchangeable then it's a
stretch to get to non-mmap data accesses also being disallowed.

Maybe the immutable metadata and mmap-only properties would only be
implied if both DAX and IMMUTABLE_META are set on a file?

Ok no more rambling until sleep. :)

--D

> 
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@xxxxxxxxxxxxx
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