Re: [RFC PATCH 2/2] mm, fs: daxfile, an interface for byte-addressable updates to pmem

On Sat, Jun 17, 2017 at 8:15 PM, Dan Williams [off-list ref] wrote:

On Sat, Jun 17, 2017 at 4:50 PM, Andy Lutomirski [off-list ref] wrote:

quoted

My other objection is that the syscall intentionally leaks a reference
to the file.  This means it needs overflow protection and it probably
shouldn't ever be allowed to use it without privilege.

We only hold the one reference while S_DAXFILE is set, so I think the
protection is there, and per Dave's original proposal this requires
CAP_LINUX_IMMUTABLE.

quoted

Why can't the underlying issue be easily fixed, though?  Could
.page_mkwrite just make sure that metadata is synced when the FS uses
DAX?

Yes, it most definitely could and that idea has been floated.

quoted

On a DAX fs, syncing metadata should be extremely fast.  This
could be conditioned on an madvise or mmap flag if performance might
be an issue.  As far as I know, this change alone should be
sufficient.

The hang up is that it requires per-fs enabling as it needs to be
careful to manage mmap_sem vs fs journal locks for example. I know the
in-development NOVA [1] filesystem is planning to support this out of
the gate. ext4 would be open to implementing it, but I think xfs is
cold on the idea. Christoph originally proposed it here [2], before
Dave went on to propose immutable semantics.

Hmm.  Given a choice between a very clean API that works without
privilege but is awkward to implement on XFS and an awkward-to-use
API, I'd personally choose the former.

Dave, even with the lock ordering issue, couldn't XFS implement
MAP_PMEM_AWARE by having .page_mkwrite work roughly like this:

if (metadata is dirty) {
  up_write(&mmap_sem);
  sync the metadata;
  down_write(&mmap_sem);
  return 0;  /* retry the fault */
} else {
  return whatever success code;
}

This might require returning VM_FAULT_RETRY instead of 0 and it might
require auditing the core mm code to make sure that it can handle
mmap_sem being dropped like this.  I don't see why it couldn't work in
principle, though.

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