Re: [PATCH v2 0/5] fs, xfs: block map immutable files for dax, dma-to-storage, and swap

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Sun, Aug 13, 2017 at 01:31:45PM -0700, Dan Williams wrote:
> On Sun, Aug 13, 2017 at 2:24 AM, Christoph Hellwig <hch@lst.de> wrote:
> > On Sat, Aug 12, 2017 at 12:19:50PM -0700, Dan Williams wrote:
> >> The application does not need to know the storage address, it needs to
> >> know that the storage address to file offset is fixed. With this
> >> information it can make assumptions about the permanence of results it
> >> gets from the kernel.
> >
> > Only if we clearly document that fact - and documenting the permanence
> > is different from saying the block map won't change.
> 
> I can get on board with that.
> 
> >
> >> For example get_user_pages() today makes no guarantees outside of
> >> "page will not be freed",
> >
> > It also makes the extremely important gurantee that the page won't
> > _move_ - e.g. that we won't do a memory migration for compaction or
> > other reasons.  That's why for example RDMA can use to register
> > memory and then we can later set up memory windows that point to this
> > registration from userspace and implement userspace RDMA.
> >
> >> but with immutable files and dax you now
> >> have a mechanism for userspace to coordinate direct access to storage
> >> addresses. Those raw storage addresses need not be exposed to the
> >> application, as you say it doesn't need to know that detail. MAP_SYNC
> >> does not fully satisfy this case because it requires agents that can
> >> generate MMU faults to coordinate with the filesystem.
> >
> > The file system is always in the fault path, can you explain what other
> > agents you are talking about?
> 
> Exactly the one's you mention below. SVM hardware can just use a
> MAP_SYNC mapping and be sure that its metadata dirtying writes are
> synchronized with the filesystem through the fault path. Hardware that
> does not have SVM, or hypervisors like Xen that want to attach their
> own static metadata about the file offset to physical block mapping,
> need a mechanism to make sure the block map is sealed while they have
> it mapped.
> 
> >> All I know is that SMB Direct for persistent memory seems like a
> >> potential consumer. I know they're not going to use a userspace
> >> filesystem or put an SMB server in the kernel.
> >
> > Last I talked to the Samba folks they didn't expect a userspace
> > SMB direct implementation to work anyway due to the fact that
> > libibverbs memory registrations interact badly with their fork()ing
> > daemon model.  That being said during the recent submission of the
> > RDMA client code some comments were made about userspace versions of
> > it, so I'm not sure if that opinion has changed in one way or another.
> 
> Ok.
> 
> >
> > Thay being said I think we absolutely should support RDMA memory
> > registrations for DAX mappings.  I'm just not sure how S_IOMAP_IMMUTABLE
> > helps with that.  We'll want a MAP_SYNC | MAP_POPULATE to make sure
> > all the blocks are polulated and all ptes are set up.  Second we need
> > to make sure get_user_page works, which for now means we'll need a
> > struct page mapping for the region (which will be really annoying
> > for PCIe mappings, like the upcoming NVMe persistent memory region),
> > and we need to gurantee that the extent mapping won't change while
> > the get_user_pages holds the pages inside it.  I think that is true
> > due to side effects even with the current DAX code, but we'll need to
> > make it explicit.  And maybe that's where we need to converge -
> > "sealing" the extent map makes sense as such a temporary measure
> > that is not persisted on disk, which automatically gets released
> > when the holding process exits, because we sort of already do this
> > implicitly.  It might also make sense to have explicitl breakable
> > seals similar to what I do for the pNFS blocks kernel server, as
> > any userspace RDMA file server would also need those semantics.
> 
> Ok, how about a MAP_DIRECT flag that arranges for faults to that range to:
> 
>     1/ only succeed if the fault can be satisfied without page cache
> 
>     2/ only install a pte for the fault if it can do so without
> triggering block map updates
> 
> So, I think it would still end up setting an inode flag to make
> xfs_bmapi_write() fail while any process has a MAP_DIRECT mapping
> active. However, it would not record that state in the on-disk
> metadata and it would automatically clear at munmap time. That should

TBH even after the last round of 'do we need this on-disk flag?' I still
wasn't 100% convinced that we really needed a permanent flag vs.
requiring apps to ask for a sealed iomap mmap like what you just
described, so I'm glad this converation has continue. :)

--D

> be enough to support the host-persistent-memory, and
> NVMe-persistent-memory use cases (provided we have struct page for
> NVMe). Although, we need more safety infrastructure in the NVMe case
> where we would need to software manage I/O coherence.
> 
> > Last but not least we have any interesting additional case for modern
> > Mellanox hardware - On Demand Paging where we don't actually do a
> > get_user_pages but the hardware implements SVM and thus gets fed
> > virtual addresses directly.  My head spins when talking about the
> > implications for DAX mappings on that, so I'm just throwing that in
> > for now instead of trying to come up with a solution.
> 
> Yeah, DAX + SVM needs more thought.
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