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

Andreas Dilger
2019-04-30T05:26:25Z

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> On Apr 29, 2019, at 10:26 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> 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?

Yes, i_op is what I was thinking.

> 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/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);
> }

This seems like kind of a hack. I guess your goal is to have ->free_inode
accessible regardless of whether the filesystem has installed its own ->i_op
methods or not, and i_fop is no longer used by this point.

That said, this seems better than increasing the size of struct inode.

> 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 *);
> + };

Cheers, Andreas

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