Re: [RFC][PATCHSET] sorting out RCU-delayed stuff in ->destroy_inode()

[Al Viro]

On Mon, Apr 29, 2019 at 10:18:04PM -0600, Andreas Dilger wrote:
> > 
> >     void                    *i_private; /* fs or device private pointer */
> > +   void (*free_inode)(struct inode *);
> 
> It seems like a waste to increase the size of every struct inode just to 
> access
> a static pointer.  Is this the only place that ->free_inode() is called?  Why
> not move the ->free_inode() pointer into inode->i_fop->free_inode() so that it
> is still directly accessible at this point.

i_op, surely?  In any case, increasing sizeof(struct inode) is not a problem -
if anything, I'd turn ->i_fop into an anon union with that.  As in,

diff --git a/Documentation/filesystems/porting 
b/Documentation/filesystems/porting
index 9d80f9e0855e..b8d3ddd8b8db 100644
--- a/Documentation/filesystems/porting
+++ b/Documentation/filesystems/porting
@@ -655,3 +655,11 @@ in your dentry operations instead.
                * if ->free_inode() is non-NULL, it gets scheduled by call_rcu()
                * combination of NULL ->destroy_inode and NULL ->free_inode is
                  treated as NULL/free_inode_nonrcu, to preserve the 
compatibility.
+
+       Note that the callback (be it via ->free_inode() or explicit call_rcu()
+       in ->destroy_inode()) is *NOT* ordered wrt superblock destruction;
+       as the matter of fact, the superblock and all associated structures
+       might be already gone.  The filesystem driver is guaranteed to be still
+       there, but that's it.  Freeing memory in the callback is fine; doing
+       more than that is possible, but requires a lot of care and is best
+       avoided.
diff --git a/fs/inode.c b/fs/inode.c
index fb45590d284e..627e1766503a 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -211,8 +211,8 @@ EXPORT_SYMBOL(free_inode_nonrcu);
 static void i_callback(struct rcu_head *head)
 {
        struct inode *inode = container_of(head, struct inode, i_rcu);
-       if (inode->i_sb->s_op->free_inode)
-               inode->i_sb->s_op->free_inode(inode);
+       if (inode->free_inode)
+               inode->free_inode(inode);
        else
                free_inode_nonrcu(inode);
 }
@@ -236,6 +236,7 @@ static struct inode *alloc_inode(struct super_block *sb)
                        if (!ops->free_inode)
                                return NULL;
                }
+               inode->free_inode = ops->free_inode;
                i_callback(&inode->i_rcu);
                return NULL;
        }
@@ -276,6 +277,7 @@ static void destroy_inode(struct inode *inode)
                if (!ops->free_inode)
                        return;
        }
+       inode->free_inode = ops->free_inode;
        call_rcu(&inode->i_rcu, i_callback);
 }
 
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 2e9b9f87caca..92732286b748 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -694,7 +694,10 @@ struct inode {
 #ifdef CONFIG_IMA
        atomic_t                i_readcount; /* struct files open RO */
 #endif
-       const struct file_operations    *i_fop; /* former 
->i_op->default_file_ops */
+       union {
+               const struct file_operations    *i_fop; /* former 
->i_op->default_file_ops */
+               void (*free_inode)(struct inode *);
+       };
        struct file_lock_context        *i_flctx;
        struct address_space    i_data;
        struct list_head        i_devices;