Re: [PATCH 6/7] mm, fs: introduce file_operations->post_mmap()

[Christoph Hellwig]

On Mon, Sep 25, 2017 at 05:14:03PM -0600, Ross Zwisler wrote:
> 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
> appropriately.

Just like in the read/write path we'll need a flag that is passed down
from the VM based on checking IS_DAX once and exactly once instead.