On Mon, Sep 25, 2017 at 04:38:45PM -0700, Dan Williams wrote:
On Mon, Sep 25, 2017 at 4:14 PM, Ross Zwisler
> When mappings are created the vma->vm_flags that they use vary based on
> whether the inode being mapped is using DAX or not. This setup happens in
> XFS via mmap_region()=>call_mmap()=>xfs_file_mmap().
> For us to be able to safely use the DAX per-inode flag we need to prevent
> S_DAX transitions when any mappings are present, and we will do that by
> looking at the address_space->i_mmap tree and returning -EBUSY if any
> mappings are present.
> Unfortunately at the time that the filesystem's file_operations->mmap()
> entry point is called the mapping has not yet been added to the
> address_space->i_mmap tree. This means that at that point in time we
> cannot determine whether or not the mapping will be set up to support DAX.
> Fix this by adding a new file_operations entry called post_mmap() which is
> called after the mapping has been added to the address_space->i_mmap tree.
> This post_mmap() op now happens at a time when we can be sure whether the
> mapping will use DAX or not, and we can set up the vma->vm_flags
> Signed-off-by: Ross Zwisler <ross.zwisler(a)linux.intel.com>
> fs/xfs/xfs_file.c | 15 ++++++++++++++-
> include/linux/fs.h | 1 +
> mm/mmap.c | 2 ++
> 3 files changed, 17 insertions(+), 1 deletion(-)
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index 2816858..9d66aaa 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -1087,9 +1087,21 @@ xfs_file_mmap(
> vma->vm_ops = &xfs_file_vm_ops;
> + return 0;
> +/* This call happens during mmap(), after the vma has been inserted into the
> + * inode->i_mapping->i_mmap tree. At this point the decision on whether or
> + * not to use DAX for this mapping has been set and will not change for the
> + * duration of the mapping.
> + */
> +STATIC void
> + struct file *filp,
> + struct vm_area_struct *vma)
> if (IS_DAX(file_inode(filp)))
> vma->vm_flags |= VM_MIXEDMAP | VM_HUGEPAGE;
It's not clear to me what this is actually protecting? vma_is_dax()
returns true regardless of the vm_flags state , so what is the benefit
to delaying the vm_flags setting to ->post_mmap()?
Right, but the point is that until the vma has been inserted into the
inode->i_mapping->i_mmap tree, the results of IS_DAX() don't matter because it
can still change. Until this insertion happens we cannot know whether or not
we should set up the vma->vm_flags to support DAX mappings (i.e. have
VM_MIXEDMAP and VM_HUGEPAGE set). This decision can only be made (in this
proposed scheme) *after* the inode->i_mapping->i_mmap tree has been
populated, which means we need another call into the filesystem after this
insertion has happened.
We don't want to mess with the existing file_operations->mmap() call because
in many filesystems that does sanity checking and setup that you really want
to have happen *before* the mapping is completed and inserted into the
Also, why is this a file_operation and not a vm_operation?
Because ->mmap() is also a file_operation, and this is an analogous call from
the mmap code that needs to happen at a different time. Or are you suggesting
that file_operations->mmap() should be moved to be a vm_operation? If not,
why would one be in one operations table and one in another?