Re: [PATCH 8/9] inode: convert per-sb inode list to a list_lru

[Jan Kara]

  Hello,

On Tue 10-03-15 15:45:23, Josef Bacik wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
> 
> The per-superblock inode list and lock is a bottleneck for systems
> that cycle inodes in and out of cache concurrently. The global lock
> is a limiting factor.
> 
> Most of the additions to the sb inode list occur on the CPU that
> allocated the inode, and most of the removals occur during evict()
> calls as a result of memory reclaim. Both of these events are local
> to the node that the inode belongs to, so it maps to the per-node
> lists that the list_lru uses.
> 
> There are several places where the inode list is walked. These can
> be converted easily to use list_lru_walk() to do their work on each
> inode on the list.
  Err, after looking at the patch I'm not so sure list_lru is such a great
fit for inode sb list. We don't use any of its LRU features so arguably
that just makes things less readable, there's some memcg code which I hope
isn't activated for the inode list but how do I tell? We just use it as
general per-numa-node linked lists so if we go down the path of
per-numa-node linked list I'd prefer to have an abstraction just for that.
Also there are some lifetime issues I'll comment on in the code.

And do we really want per-numa-node lists? Why not per-cpu? Is it the
memory cost we are concerned about? Or is it the cost of iterating all CPUs
when wanting to traverse all inodes? The changelog doesn't tell. The
traversal is used on unmount (inode eviction, fsnotify mark eviction),
quota on/off (adding / removing dquot pointers to / from inodes),
drop_caches, sync (iterate all block devices). Sync is IMO the only one
where CPU overhead might be a concern but we could create a better method
for iterating all block devices. After all creating / removing block
devices could happily use just ordinary lists. So CPU overhead shouldn't be
a huge problem. Memory overhead is (list_head + spinlock) per-sb-per-cpu.
That doesn't sound too terrible either.

When speaking about this with Andi Kleen, he was suggesting that we maybe
want to not have a per-cpu lists but a list per a batch of CPUs (say 8, the
actual number would be tuned during boot based on the number of actual
CPUs). That may be an option as well.

I wouldn't like to block this series just for this patch since all the
others are fine (modulo some minor comments). So could you repost the
series without this patch so that it can get queued for the coming merge
window? It should be easy to rebase the patch 9/9 to not need this patch.

> diff --git a/fs/block_dev.c b/fs/block_dev.c
> index 2eb2436..d23ce6f 100644
> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -1749,38 +1749,56 @@ int __invalidate_device(struct block_device *bdev, bool kill_dirty)
>  }
>  EXPORT_SYMBOL(__invalidate_device);
>  
> -void iterate_bdevs(void (*func)(struct block_device *, void *), void *arg)
> -{
> -	struct inode *inode, *old_inode = NULL;
> +struct bdev_iter {
> +	void (*func)(struct block_device *, void *);
> +	void *arg;
> +	struct inode *toput_inode;
> +};
>  
> -	spin_lock(&blockdev_superblock->s_inode_list_lock);
> -	list_for_each_entry(inode, &blockdev_superblock->s_inodes, i_sb_list) {
> -		struct address_space *mapping = inode->i_mapping;
> +static enum lru_status
> +bdev_iter_cb(struct list_head *item, struct list_lru_one *lru,
> +	     spinlock_t *lock, void *cb_arg)
> +{
> +	struct bdev_iter *iter = cb_arg;
> +	struct inode *inode = container_of(item, struct inode, i_sb_list);
>  
> -		spin_lock(&inode->i_lock);
> -		if (inode->i_state & (I_FREEING|I_WILL_FREE|I_NEW) ||
> -		    mapping->nrpages == 0) {
> -			spin_unlock(&inode->i_lock);
> -			continue;
> -		}
> -		__iget(inode);
> +	spin_lock(&inode->i_lock);
> +	if (inode->i_state & (I_FREEING|I_WILL_FREE|I_NEW) ||
> +	    inode->i_mapping->nrpages == 0) {
>  		spin_unlock(&inode->i_lock);
> -		spin_unlock(&blockdev_superblock->s_inode_list_lock);
> -		/*
> -		 * We hold a reference to 'inode' so it couldn't have been
> -		 * removed from s_inodes list while we dropped the
> -		 * s_inode_list_lock  We cannot iput the inode now as we can
> -		 * be holding the last reference and we cannot iput it under
> -		 * s_inode_list_lock. So we keep the reference and iput it
> -		 * later.
> -		 */
> -		iput(old_inode);
> -		old_inode = inode;
> +		return LRU_SKIP;
> +	}
> +	__iget(inode);
> +	spin_unlock(&inode->i_lock);
> +	spin_unlock(lock);
>  
> -		func(I_BDEV(inode), arg);
> +	iput(iter->toput_inode);
> +	iter->toput_inode = inode;
>  
> -		spin_lock(&blockdev_superblock->s_inode_list_lock);
> -	}
> -	spin_unlock(&blockdev_superblock->s_inode_list_lock);
> -	iput(old_inode);
> +	iter->func(I_BDEV(inode), iter->arg);
> +
> +	/*
> +	 * Even though we have dropped the lock here, we can return LRU_SKIP as
> +	 * we have a reference to the current inode and so it's next pointer is
> +	 * guaranteed to be valid even though we dropped the list lock.
> +	 */
> +	spin_lock(lock);
> +	return LRU_SKIP;
> +}
  So I actually think doing the iteration like this is buggy with list_lru
code. When we pin inode by grabbing i_count, we are only sure the inode
won't go away under us. However there's nothing which protects from list
modifications. So this method is only safe to be combined with
list_for_each[_entry](). Specifically it does *not* work when combined with
list_for_each_safe() which __list_lru_walk_one() uses because the next
pointer that is cached may become invalid once we drop the lock. And I'd
really hate to try to make these two work together - that would seem really
fragile since subtle changes to how lru_list iteration work could break
inode iteration.

This is true for all the iterations over the inodes that are playing with
i_count. It's not just about this particular case of blockdev iteration.

> diff --git a/fs/notify/inode_mark.c b/fs/notify/inode_mark.c
> index a4e1a8f..a0cdc66 100644
> --- a/fs/notify/inode_mark.c
> +++ b/fs/notify/inode_mark.c
> @@ -161,87 +161,60 @@ int fsnotify_add_inode_mark(struct fsnotify_mark *mark,
>  	return ret;
>  }
>  
> -/**
> - * fsnotify_unmount_inodes - an sb is unmounting.  handle any watched inodes.
> - * @sb: superblock being unmounted.
> - *
> - * Called during unmount with no locks held, so needs to be safe against
> - * concurrent modifiers. We temporarily drop sb->s_inode_list_lock and CAN block.
> - */
> -void fsnotify_unmount_inodes(struct super_block *sb)
> -{
> -	struct inode *inode, *next_i, *need_iput = NULL;
> -
> -	spin_lock(&sb->s_inode_list_lock);
> -	list_for_each_entry_safe(inode, next_i, &sb->s_inodes, i_sb_list) {
> -		struct inode *need_iput_tmp;
> -
> -		/*
> -		 * We cannot __iget() an inode in state I_FREEING,
> -		 * I_WILL_FREE, or I_NEW which is fine because by that point
> -		 * the inode cannot have any associated watches.
> -		 */
> -		spin_lock(&inode->i_lock);
> -		if (inode->i_state & (I_FREEING|I_WILL_FREE|I_NEW)) {
> -			spin_unlock(&inode->i_lock);
> -			continue;
> -		}
> +static enum lru_status
> +fsnotify_unmount_inode(struct list_head *item, struct list_lru_one *lru,
> +		       spinlock_t *lock, void *cb_arg)
> + {
> +	struct inode **toput_inode = cb_arg;
> +	struct inode *inode = container_of(item, struct inode, i_sb_list);
> +
> +	/* New or being freed inodes cannot have any associated watches. */
> +	spin_lock(&inode->i_lock);
> +	if (inode->i_state & (I_FREEING|I_WILL_FREE|I_NEW)) {
> +		spin_unlock(&inode->i_lock);
> +		return LRU_SKIP;
> +	}
>  
> -		/*
> -		 * If i_count is zero, the inode cannot have any watches and
> -		 * doing an __iget/iput with MS_ACTIVE clear would actually
> -		 * evict all inodes with zero i_count from icache which is
> -		 * unnecessarily violent and may in fact be illegal to do.
> -		 */
> -		if (!atomic_read(&inode->i_count)) {
> -			spin_unlock(&inode->i_lock);
> -			continue;
> -		}
> -
> -		need_iput_tmp = need_iput;
> -		need_iput = NULL;
> -
> -		/* In case fsnotify_inode_delete() drops a reference. */
> -		if (inode != need_iput_tmp)
> -			__iget(inode);
> -		else
> -			need_iput_tmp = NULL;
> +	/* If i_count is zero, the inode cannot have any watches */
> +	if (!atomic_read(&inode->i_count)) {
>  		spin_unlock(&inode->i_lock);
> +		return LRU_SKIP;
> +	}
>  
> -		/* In case the dropping of a reference would nuke next_i. */
> -		while (&next_i->i_sb_list != &sb->s_inodes) {
> -			spin_lock(&next_i->i_lock);
> -			if (!(next_i->i_state & (I_FREEING | I_WILL_FREE)) &&
> -						atomic_read(&next_i->i_count)) {
> -				__iget(next_i);
> -				need_iput = next_i;
> -				spin_unlock(&next_i->i_lock);
> -				break;
> -			}
> -			spin_unlock(&next_i->i_lock);
> -			next_i = list_entry(next_i->i_sb_list.next,
> -						struct inode, i_sb_list);
> -		}
> +	__iget(inode);
> +	spin_unlock(&inode->i_lock);
> +	spin_unlock(lock);
>  
> -		/*
> -		 * We can safely drop s_inode_list_lock here because either
> -		 * we actually hold references on both inode and next_i or
> -		 * end of list.  Also no new inodes will be added since the
> -		 * umount has begun.
> -		 */
> -		spin_unlock(&sb->s_inode_list_lock);
> +	iput(*toput_inode);
> +	*toput_inode = inode;
>  
> -		if (need_iput_tmp)
> -			iput(need_iput_tmp);
> +	/* for each watch, send FS_UNMOUNT and then remove it */
> +	fsnotify(inode, FS_UNMOUNT, inode, FSNOTIFY_EVENT_INODE, NULL, 0);
> +	fsnotify_inode_delete(inode);
>  
> -		/* for each watch, send FS_UNMOUNT and then remove it */
> -		fsnotify(inode, FS_UNMOUNT, inode, FSNOTIFY_EVENT_INODE, NULL, 0);
> +	/*
> +	 * Even though we have dropped the lock here, we can return LRU_SKIP as
> +	 * we have a reference to the current inode and so it's next pointer is
> +	 * guaranteed to be valid even though we dropped the list lock.
> +	 */
> +	spin_lock(lock);
> +	return LRU_SKIP;
> +}
  So in the original code of fsnotify_unmount_inodes() you can see the
hoops you have to jump through when having to iterate inode list with 
list_for_each_entry_safe(). Deleting that code without deeper thought
wasn't a good idea ;).

As a side note, it should be possible to convert this code to use just
list_for_each() and avoid all that crap but it will require me to dwelve into
fsnotify magic again to verify it's correct. I guess I'll leave that after
your patches are queued so that I don't make life harder to you.

								Honza
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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