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

Al Viro
2019-04-30T04:26:29Z

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;