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

[Ross Zwisler]

On Mon, Sep 25, 2017 at 04:38:45PM -0700, Dan Williams wrote:
> On Mon, Sep 25, 2017 at 4:14 PM, Ross Zwisler
> <ross.zwisler@xxxxxxxxxxxxxxx> 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.
> >
> > Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> > ---
> >  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(
> >  {
> >         file_accessed(filp);
> >         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
> > +xfs_file_post_mmap(
> > +       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
inode->i_mapping->i_mmap tree.

> 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?