Re: [Qemu-devel] [PATCH v4 5/5] xfs: disable map_sync for async flush

[Jan Kara]

On Thu 04-04-19 05:09:10, Pankaj Gupta wrote:
> 
> > > On Thu, Apr 04, 2019 at 09:09:12AM +1100, Dave Chinner wrote:
> > > > On Wed, Apr 03, 2019 at 04:10:18PM +0530, Pankaj Gupta wrote:
> > > > > Virtio pmem provides asynchronous host page cache flush
> > > > > mechanism. we don't support 'MAP_SYNC' with virtio pmem
> > > > > and xfs.
> > > > > 
> > > > > Signed-off-by: Pankaj Gupta <address@hidden>
> > > > > ---
> > > > >  fs/xfs/xfs_file.c | 8 ++++++++
> > > > >  1 file changed, 8 insertions(+)
> > > > > 
> > > > > diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> > > > > index 1f2e2845eb76..dced2eb8c91a 100644
> > > > > --- a/fs/xfs/xfs_file.c
> > > > > +++ b/fs/xfs/xfs_file.c
> > > > > @@ -1203,6 +1203,14 @@ xfs_file_mmap(
> > > > >       if (!IS_DAX(file_inode(filp)) && (vma->vm_flags & VM_SYNC))
> > > > >               return -EOPNOTSUPP;
> > > > >  
> > > > > +     /* We don't support synchronous mappings with DAX files if
> > > > > +      * dax_device is not synchronous.
> > > > > +      */
> > > > > +     if (IS_DAX(file_inode(filp)) && !dax_synchronous(
> > > > > +             xfs_find_daxdev_for_inode(file_inode(filp))) &&
> > > > > +                                     (vma->vm_flags & VM_SYNC))
> > > > > +             return -EOPNOTSUPP;
> > > > > +
> > > > >       file_accessed(filp);
> > > > >       vma->vm_ops = &xfs_file_vm_ops;
> > > > >       if (IS_DAX(file_inode(filp)))
> > > > 
> > > > All this ad hoc IS_DAX conditional logic is getting pretty nasty.
> > > > 
> > > > xfs_file_mmap(
> > > > ....
> > > > {
> > > >         struct inode    *inode = file_inode(filp);
> > > > 
> > > >         if (vma->vm_flags & VM_SYNC) {
> > > >                 if (!IS_DAX(inode))
> > > >                         return -EOPNOTSUPP;
> > > >                 if (!dax_synchronous(xfs_find_daxdev_for_inode(inode))
> > > >                         return -EOPNOTSUPP;
> > > >         }
> > > > 
> > > >         file_accessed(filp);
> > > >         vma->vm_ops = &xfs_file_vm_ops;
> > > >         if (IS_DAX(inode))
> > > >                 vma->vm_flags |= VM_HUGEPAGE;
> > > >         return 0;
> > > > }
> > > > 
> > > > 
> > > > Even better, factor out all the "MAP_SYNC supported" checks into a
> > > > helper so that the filesystem code just doesn't have to care about
> > > > the details of checking for DAX+MAP_SYNC support....
> > > 
> > > Seconded, since ext4 has nearly the same flag validation logic.
> > 
> 
> Only issue with this I see is we need the helper function only for supported
> filesystems ext4 & xfs (right now). If I create the function in "fs.h" it 
> will be compiled for every filesystem, even for those don't need it.  
> 
> Sample patch below, does below patch is near to what you have in mind?

So I would put the helper in include/linux/dax.h and have it like:

bool daxdev_mapping_supported(struct vm_area_struct *vma,
                              struct dax_device *dax_dev)
{
        if (!(vma->vm_flags & VM_SYNC))
                return true;
        if (!IS_DAX(file_inode(vma->vm_file)))
                return false;
        return dax_synchronous(dax_dev);
}

                                                                Honza
> 
> =================
> 
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index 1f2e2845eb76..614995170cac 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -1196,12 +1196,17 @@ xfs_file_mmap(
>         struct file     *filp,
>         struct vm_area_struct *vma)
>  {
> +       struct dax_device *dax_dev = 
> xfs_find_daxdev_for_inode(file_inode(filp));
> +
>         /*
> -        * We don't support synchronous mappings for non-DAX files. At least
> -        * until someone comes with a sensible use case.
> +        * We don't support synchronous mappings for non-DAX files and
> +        * for DAX files if underneath dax_device is not synchronous.
>          */
> -       if (!IS_DAX(file_inode(filp)) && (vma->vm_flags & VM_SYNC))
> -               return -EOPNOTSUPP;
> +       if (vma->vm_flags & VM_SYNC) {
> +               int err = is_synchronous(filp, dax_dev);
> +               if (err)
> +                       return err;
> +       }
>  
>         file_accessed(filp);
>         vma->vm_ops = &xfs_file_vm_ops;
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 8b42df09b04c..add017de3dd7 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2162,6 +2162,20 @@ static inline void file_accessed(struct file *file)
>                 touch_atime(&file->f_path);
>  }
>  
> +struct dax_device;
> +extern bool dax_synchronous(struct dax_device *dax_dev);
> +static inline int is_synchronous(struct file *filp, struct dax_device 
> *dax_dev)
> +{
> +       struct inode *inode = file_inode(filp);
> +
> +       if (!IS_DAX(inode))
> +               return -EOPNOTSUPP;
> +       if (!dax_synchronous(dax_dev))
> +               return -EOPNOTSUPP;
> +
> +       return 0;
> +}
> +
>  int sync_inode(struct inode *inode, struct writeback_control *wbc);
>  int sync_inode_metadata(struct inode *inode, int wait);
>  
> ---------
> 
> Thanks,
> Pankaj
> 
> 
> 
-- 
Jan Kara <address@hidden>
SUSE Labs, CR