Re: [PATCH 3/3] btrfs: Perform locking/unlocking in btrfs_remap_file_range()

[Goldwyn Rodrigues <rgoldwyn@suse.de>]

On 12:08 26/02, Filipe Manana wrote:
> On Mon, Feb 25, 2019 at 7:08 PM Goldwyn Rodrigues <rgoldwyn@suse.de> wrote:
> >
> > From: Goldwyn Rodrigues <rgoldwyn@suse.com>
> >
> > Moves code to make it more readable, so as locking and unlocking is
> > done in the same function.
> >
> > Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
> > ---
> >  fs/btrfs/ioctl.c | 53 +++++++++++++++++++++--------------------------------
> >  1 file changed, 21 insertions(+), 32 deletions(-)
> >
> > diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
> > index 9c8e1734429c..f0ae1af91ff3 100644
> > --- a/fs/btrfs/ioctl.c
> > +++ b/fs/btrfs/ioctl.c
> > @@ -3975,22 +3975,6 @@ static int btrfs_remap_file_range_prep(struct file *file_in, loff_t pos_in,
> >         u64 wb_len;
> >         int ret;
> >
> > -       if (!(remap_flags & REMAP_FILE_DEDUP)) {
> > -               struct btrfs_root *root_out = BTRFS_I(inode_out)->root;
> > -
> > -               if (btrfs_root_readonly(root_out))xfs_reflink_remap_prep
> > -                       return -EROFS;
> > -
> > -               if (file_in->f_path.mnt != file_out->f_path.mnt ||
> > -                   inode_in->i_sb != inode_out->i_sb)
> > -                       return -EXDEV;
> > -       }
> 
> Why move these checks?
> The goal of the _prep function (both btrfs and vfs)  is to have the
> checks for all needed conditions in one place.

In the original flow, these checks were done without locks.
But I suppose they can be done with locks held as well.

> 
> As for the lock/unlock, it follows the same pattern from xfs
> (xfs_reflink_remap_prep and xfs_file_remap_range).
> No complaints about changing this, I'm just neutral about it.
> 

I just read the xfs code and yes it is similar. Locking and unlocking
in separate functions makes it difficult to read, especially
when it can be done in the same function.

> > -
> > -       if (same_inode)
> > -               inode_lock(inode_in);
> > -       else
> > -               btrfs_double_inode_lock(inode_in, inode_out);
> > -
> >         /*
> >          * Now that the inodes are locked, we need to start writeback ourselves
> >          * and can not rely on the writeback from the VFS's generic helper
> > @@ -4022,26 +4006,14 @@ static int btrfs_remap_file_range_prep(struct file *file_in, loff_t pos_in,
> >         ret = btrfs_wait_ordered_range(inode_in, ALIGN_DOWN(pos_in, bs),
> >                                        wb_len);
> >         if (ret < 0)
> > -               goto out_unlock;
> > +               return ret;
> >         ret = btrfs_wait_ordered_range(inode_out, ALIGN_DOWN(pos_out, bs),
> >                                        wb_len);
> >         if (ret < 0)
> > -               goto out_unlock;
> > +               return ret;
> >
> > -       ret = generic_remap_file_range_prep(file_in, pos_in, file_out, pos_out,
> > +       return generic_remap_file_range_prep(file_in, pos_in, file_out, pos_out,
> >                                             len, remap_flags);
> > -       if (ret < 0 || *len == 0)
> > -               goto out_unlock;
> > -
> > -       return 0;
> > -
> > - out_unlock:
> > -       if (same_inode)
> > -               inode_unlock(inode_in);
> > -       else
> > -               btrfs_double_inode_unlock(inode_in, inode_out);
> > -
> > -       return ret;
> >  }
> >
> >  loff_t btrfs_remap_file_range(struct file *src_file, loff_t off,
> > @@ -4056,16 +4028,33 @@ loff_t btrfs_remap_file_range(struct file *src_file, loff_t off,
> >         if (remap_flags & ~(REMAP_FILE_DEDUP | REMAP_FILE_ADVISORY))
> >                 return -EINVAL;
> >
> > +       if (!(remap_flags & REMAP_FILE_DEDUP)) {
> > +               struct btrfs_root *root_out = BTRFS_I(dst_inode)->root;
> > +
> > +               if (btrfs_root_readonly(root_out))
> > +                       return -EROFS;
> > +
> > +               if (src_file->f_path.mnt != dst_file->f_path.mnt ||
> > +                   src_inode->i_sb != dst_inode->i_sb)
> > +                       return -EXDEV;
> > +       }
> > +
> > +       if (same_inode)
> > +               inode_lock(src_inode);
> > +       else
> > +               btrfs_double_inode_lock(src_inode, dst_inode);
> > +
> >         ret = btrfs_remap_file_range_prep(src_file, off, dst_file, destoff,
> >                                           &len, remap_flags);
> >         if (ret < 0 || len == 0)
> > -               return ret;
> > +               goto out_unlock;
> >
> >         if (remap_flags & REMAP_FILE_DEDUP)
> >                 ret = btrfs_extent_same(src_inode, off, len, dst_inode, destoff);
> >         else
> >                 ret = btrfs_clone_files(dst_file, src_file, off, len, destoff);
> >
> > +out_unlock:
> >         if (same_inode)
> >                 inode_unlock(src_inode);
> >         else
> > --
> > 2.16.4
> >
> 
> 
> -- 
> Filipe David Manana,
> 
> “Whether you think you can, or you think you can't — you're right.”
> 

-- 
Goldwyn