linux-kernel - Re: [RFC PATCH V2 08/12] fs/xfs: Add lock/unlock mode to xfs

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Date:   Tue, 14 Jan 2020 16:57:28 -0800
From:   Ira Weiny <ira.weiny@...el.com>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     linux-kernel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Christoph Hellwig <hch@....de>,
        "Theodore Y. Ts'o" <tytso@....edu>, Jan Kara <jack@...e.cz>,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [RFC PATCH V2 08/12] fs/xfs: Add lock/unlock mode to xfs

On Mon, Jan 13, 2020 at 04:35:21PM -0800, 'Ira Weiny' wrote:
> On Mon, Jan 13, 2020 at 02:19:57PM -0800, Darrick J. Wong wrote:
> > On Fri, Jan 10, 2020 at 11:29:38AM -0800, ira.weiny@...el.com wrote:
> > > From: Ira Weiny <ira.weiny@...el.com>
> 
> [snip]
> 
> > >  
> > > diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
> > > index 401da197f012..e8fd95b75e5b 100644
> > > --- a/fs/xfs/xfs_inode.c
> > > +++ b/fs/xfs/xfs_inode.c
> > > @@ -142,12 +142,12 @@ xfs_ilock_attr_map_shared(
> > >   *
> > >   * Basic locking order:
> > >   *
> > > - * i_rwsem -> i_mmap_lock -> page_lock -> i_ilock
> > > + * i_rwsem -> i_dax_sem -> i_mmap_lock -> page_lock -> i_ilock
> > 
> > Mmmmmm, more locks.  Can we skip the extra lock if CONFIG_FSDAX=n or if
> > the filesystem devices don't support DAX at all?
> 
> I'll look into it.
> 
> > 
> > Also, I don't think we're actually following the i_rwsem -> i_daxsem
> > order in fallocate, and possibly elsewhere too?
> 
> I'll have to verify.  It took a lot of iterations to get the order working so
> I'm not going to claim perfection.

Yes this was inconsistent.  The code was right WRT i_rwsem.

mmap_sem may have issues:

What about this?

diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
index c5d11b70d067..8808782a085e 100644
--- a/fs/xfs/xfs_inode.c
+++ b/fs/xfs/xfs_inode.c
@@ -142,12 +142,12 @@ xfs_ilock_attr_map_shared(
  *
  * Basic locking order:
  *
- * i_rwsem -> i_dax_sem -> i_mmap_lock -> page_lock -> i_ilock
+ * i_dax_sem -> i_rwsem -> i_mmap_lock -> page_lock -> i_ilock
  *
  * mmap_sem locking order:
  *
  * i_rwsem -> page lock -> mmap_sem
- * mmap_sem -> i_dax_sem -> i_mmap_lock -> page_lock
+ * i_dax_sem -> mmap_sem -> i_mmap_lock -> page_lock
  *
  * The difference in mmap_sem locking order mean that we cannot hold the
  * i_mmap_lock over syscall based read(2)/write(2) based IO. These IO paths can
diff --git a/mm/mmap.c b/mm/mmap.c
index e6b68924b7ca..b500aef30b27 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1547,18 +1547,12 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
                        vm_flags |= VM_NORESERVE;
        }
 
-       if (file)
-               lock_inode_mode(file_inode(file));
-
        addr = mmap_region(file, addr, len, vm_flags, pgoff, uf);
        if (!IS_ERR_VALUE(addr) &&
            ((vm_flags & VM_LOCKED) ||
             (flags & (MAP_POPULATE | MAP_NONBLOCK)) == MAP_POPULATE))
                *populate = len;
 
-       if (file)
-               unlock_inode_mode(file_inode(file));
-
        return addr;
 }
 
diff --git a/mm/util.c b/mm/util.c
index 988d11e6c17c..1cfead8cd1ce 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -501,11 +501,18 @@ unsigned long vm_mmap_pgoff(struct file *file, unsigned long addr,
 
        ret = security_mmap_file(file, prot, flag);
        if (!ret) {
-               if (down_write_killable(&mm->mmap_sem))
+               if (file)
+                       lock_inode_mode(file_inode(file));
+               if (down_write_killable(&mm->mmap_sem)) {
+                       if (file)
+                               unlock_inode_mode(file_inode(file));
                        return -EINTR;
+               }
                ret = do_mmap_pgoff(file, addr, len, prot, flag, pgoff,
                                    &populate, &uf);
                up_write(&mm->mmap_sem);
+               if (file)
+                       unlock_inode_mode(file_inode(file));
                userfaultfd_unmap_complete(mm, &uf);
                if (populate)
                        mm_populate(ret, populate);