On Fri, Oct 06, 2017 at 03:35:49PM -0700, Dan Williams wrote:
> 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 <jack@xxxxxxx>
> Cc: Arnd Bergmann <arnd@xxxxxxxx>
> Cc: Jeff Moyer <jmoyer@xxxxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxx>
> Cc: Dave Chinner <david@xxxxxxxxxxxxx>
> Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: "Darrick J. Wong" <darrick.wong@xxxxxxxxxx>
> Cc: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> Cc: Jeff Layton <jlayton@xxxxxxxxxxxxxxx>
> Cc: "J. Bruce Fields" <bfields@xxxxxxxxxxxx>
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> ---
> 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.
>
> /*
> * 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?
> +{
> + 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?
> +
> + 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....
.....
>
> +#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?
> + 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...
Cheers,
Dave.
--
Dave Chinner
david@xxxxxxxxxxxxx
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