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    SubjectRe: [RFC PATCH V2 08/12] fs/xfs: Add lock/unlock mode to xfs
    On Thu, Jan 16, 2020 at 10:24:46AM +0100, Jan Kara wrote:
    > On Fri 10-01-20 11:29:38, ira.weiny@intel.com wrote:
    > > From: Ira Weiny <ira.weiny@intel.com>
    > >
    > > XFS requires regular files to be locked while changing to/from DAX mode.
    > >
    > > Define a new DAX lock type and implement the [un]lock_mode() inode
    > > operation callbacks.
    > >
    > > We define a new XFS_DAX_* lock type to carry the lock through the
    > > transaction because we don't want to use IOLOCK as that would cause
    > > performance issues with locking of the inode itself.
    > >
    > > Signed-off-by: Ira Weiny <ira.weiny@intel.com>
    > ...
    > > diff --git a/fs/xfs/xfs_inode.h b/fs/xfs/xfs_inode.h
    > > index 492e53992fa9..693ca66bd89b 100644
    > > --- a/fs/xfs/xfs_inode.h
    > > +++ b/fs/xfs/xfs_inode.h
    > > @@ -67,6 +67,9 @@ typedef struct xfs_inode {
    > > spinlock_t i_ioend_lock;
    > > struct work_struct i_ioend_work;
    > > struct list_head i_ioend_list;
    > > +
    > > + /* protect changing the mode to/from DAX */
    > > + struct percpu_rw_semaphore i_dax_sem;
    > > } xfs_inode_t;
    >
    > This adds overhead of ~32k per inode for typical distro kernel.

    Wow!

    > That's not
    > going to fly.

    Probably not...

    > That's why ext4 has similar kind of lock in the superblock
    > shared by all inodes. For read side it does not matter because that's
    > per-cpu and shared lock. For write side we don't care as changing inode
    > access mode should be rare.

    Sounds reasonable to me. I'll convert it.

    Thanks for pointing this out, that would have been bad indeed.
    Ira

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