On Sun, Oct 8, 2017 at 8:40 PM, Dave Chinner [off-list ref] wrote:

Thanks for the review Dave.

On Fri, Oct 06, 2017 at 03:35:49PM -0700, Dan Williams wrote:

quoted

MAP_DIRECT is an mmap(2) flag with the following semantics:

  MAP_DIRECT
  When specified with MAP_SHARED_VALIDATE, sets up a file lease with the
  same lifetime as the mapping. Unlike a typical F_RDLCK lease this lease
  is broken when a "lease breaker" attempts to write(2), change the block
  map (fallocate), or change the size of the file. Otherwise the mechanism
  of a lease break is identical to the typical lease break case where the
  lease needs to be removed (munmap) within the number of seconds
  specified by /proc/sys/fs/lease-break-time. If the lease holder fails to
  remove the lease in time the kernel will invalidate the mapping and
  force all future accesses to the mapping to trigger SIGBUS.

  In addition to lease break timeouts causing faults in the mapping to
  result in SIGBUS, other states of the file will trigger SIGBUS at fault
  time:

      * The file is not DAX capable
      * The file has reflinked (copy-on-write) blocks
      * The fault would trigger the filesystem to allocate blocks
      * The fault would trigger the filesystem to perform extent conversion

  In other words, MAP_DIRECT expects and enforces a fully allocated file
  where faults can be satisfied without modifying block map metadata.

  An unprivileged process may establish a MAP_DIRECT mapping on a file
  whose UID (owner) matches the filesystem UID of the  process. A process
  with the CAP_LEASE capability may establish a MAP_DIRECT mapping on
  arbitrary files

  ERRORS
  EACCES Beyond the typical mmap(2) conditions that trigger EACCES
  MAP_DIRECT also requires the permission to set a file lease.

  EOPNOTSUPP The filesystem explicitly does not support the flag

  SIGBUS Attempted to write a MAP_DIRECT mapping at a file offset that
         might require block-map updates, or the lease timed out and the
         kernel invalidated the mapping.

Cc: Jan Kara <redacted>
Cc: Arnd Bergmann <redacted>
Cc: Jeff Moyer <redacted>
Cc: Christoph Hellwig <redacted>
Cc: Dave Chinner <redacted>
Cc: Alexander Viro <viro-RmSDqhL/yNMiFSDQTTA3OLVCufUGDwFn@public.gmane.org>
Cc: "Darrick J. Wong" <redacted>
Cc: Ross Zwisler <ross.zwisler-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>
Cc: Jeff Layton <redacted>
Cc: "J. Bruce Fields" <redacted>
Signed-off-by: Dan Williams <redacted>
---
 fs/xfs/Kconfig                  |    2 -
 fs/xfs/xfs_file.c               |  102 +++++++++++++++++++++++++++++++++++++++
 include/linux/mman.h            |    3 +
 include/uapi/asm-generic/mman.h |    1
 4 files changed, 106 insertions(+), 2 deletions(-)

diff --git a/fs/xfs/Kconfig b/fs/xfs/Kconfig
index f62fc6629abb..f8765653a438 100644
--- a/fs/xfs/Kconfig
+++ b/fs/xfs/Kconfig

@@ -112,4 +112,4 @@ config XFS_ASSERT_FATAL

 config XFS_LAYOUT
      def_bool y
-     depends on EXPORTFS_BLOCK_OPS
+     depends on EXPORTFS_BLOCK_OPS || FS_DAX

diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index ebdd0bd2b261..e35518600e28 100644
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c

@@ -40,12 +40,22 @@
 #include "xfs_iomap.h"
 #include "xfs_reflink.h"

+#include <linux/mman.h>
 #include <linux/dcache.h>
 #include <linux/falloc.h>
 #include <linux/pagevec.h>
+#include <linux/mapdirect.h>
 #include <linux/backing-dev.h>

 static const struct vm_operations_struct xfs_file_vm_ops;
+static const struct vm_operations_struct xfs_file_vm_direct_ops;
+
+static inline bool
+is_xfs_map_direct(
+             struct vm_area_struct *vma)
+{
+     return vma->vm_ops == &xfs_file_vm_direct_ops;
+}

Namespacing (xfs_vma_is_direct) and whitespace damage.

Will fix.

quoted

 /*
  * Clear the specified ranges to zero through either the pagecache or DAX.

@@ -1008,6 +1018,26 @@ xfs_file_llseek(
      return vfs_setpos(file, offset, inode->i_sb->s_maxbytes);
 }

+static int
+xfs_vma_checks(
+     struct vm_area_struct   *vma,
+     struct inode            *inode)

Exactly what are we checking for - function name doesn't tell me,
and there's no comments, either?

Ok, I'll improve this.

quoted

+{
+     if (!is_xfs_map_direct(vma))
+             return 0;
+
+     if (!is_map_direct_valid(vma->vm_private_data))
+             return VM_FAULT_SIGBUS;
+
+     if (xfs_is_reflink_inode(XFS_I(inode)))
+             return VM_FAULT_SIGBUS;
+
+     if (!IS_DAX(inode))
+             return VM_FAULT_SIGBUS;

And how do we get is_xfs_map_direct() set to true if we don't have a
DAX inode or the inode has shared extents?

So, this was my way of trying to satisfy the request you made here:

    https://lkml.org/lkml/2017/8/11/876

i.e. allow MAP_DIRECT on non-dax files to enable a use case of
freezing the block-map to examine which file extents are linked. If
you don't want to use MAP_DIRECT for this, we can move these checks to
mmap time.

quoted

+
+     return 0;
+}
+
 /*
  * Locking for serialisation of IO during page faults. This results in a lock
  * ordering of:

@@ -1024,6 +1054,7 @@ __xfs_filemap_fault(
      enum page_entry_size    pe_size,
      bool                    write_fault)
 {
+     struct vm_area_struct   *vma = vmf->vma;
      struct inode            *inode = file_inode(vmf->vma->vm_file);

You missed this vmf->vma....

.....

quoted

+#define XFS_MAP_SUPPORTED (LEGACY_MAP_MASK | MAP_DIRECT)
+
+STATIC int
+xfs_file_mmap_validate(
+     struct file             *filp,
+     struct vm_area_struct   *vma,
+     unsigned long           map_flags,
+     int                     fd)
+{
+     struct inode            *inode = file_inode(filp);
+     struct xfs_inode        *ip = XFS_I(inode);
+     struct map_direct_state *mds;
+
+     if (map_flags & ~(XFS_MAP_SUPPORTED))
+             return -EOPNOTSUPP;
+
+     if ((map_flags & MAP_DIRECT) == 0)
+             return xfs_file_mmap(filp, vma);
+
+     file_accessed(filp);
+     vma->vm_ops = &xfs_file_vm_direct_ops;
+     if (IS_DAX(inode))
+             vma->vm_flags |= VM_MIXEDMAP | VM_HUGEPAGE;

And if it isn't a DAX inode? what is MAP_DIRECT supposed to do then?

In the non-DAX case it just takes the FL_LAYOUT file lease... although
we could also just have an fcntl for that purpose. The use case of
just freezing the block map does not need a mapping.

quoted

+     mds = map_direct_register(fd, vma);
+     if (IS_ERR(mds))
+             return PTR_ERR(mds);
+
+     /* flush in-flight faults */
+     xfs_ilock(ip, XFS_MMAPLOCK_EXCL);
+     xfs_iunlock(ip, XFS_MMAPLOCK_EXCL);

Urk. That's nasty. And why is it even necessary? Please explain why
this is necessary in the comment, because it's not at all obvious to
me...

This is related to your other observation about i_mapdcount and adding
an iomap_can_allocate() helper. I think I can clean both of these up
by using a call to break_layout(inode, false) and bailing in
->iomap_begin() if it returns EWOULDBLOCK. This would also fix the
current problem that allocating write-faults don't start the lease
break process.