linux-kernel - Re: [PATCH] vfs: drop one lock trip in evict()

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Message-ID: <20240826202746.ipovnb5hfom7jkmb@quack3>
Date: Mon, 26 Aug 2024 22:27:46 +0200
From: Jan Kara <jack@...e.cz>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: brauner@...nel.org, viro@...iv.linux.org.uk, jack@...e.cz,
	jlayton@...nel.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] vfs: drop one lock trip in evict()

On Tue 13-08-24 16:36:26, Mateusz Guzik wrote:
> Most commonly neither I_LRU_ISOLATING nor I_SYNC are set, but the stock
> kernel takes a back-to-back relock trip to check for them.
> 
> It probably can be avoided altogether, but for now massage things back
> to just one lock acquire.
> 
> Signed-off-by: Mateusz Guzik <mjguzik@...il.com>

Back from vacation so not sure if this is still actual but the patch looks
good to me. Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> ---
> 
> there are smp_mb's in the area I'm going to look at removing at some
> point(tm), in the meantime I think this is an easy cleanup
> 
> has a side effect of whacking a inode_wait_for_writeback which was only
> there to deal with not holding the lock
> 
>  fs/fs-writeback.c | 17 +++--------------
>  fs/inode.c        |  5 +++--
>  2 files changed, 6 insertions(+), 16 deletions(-)
> 
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 4451ecff37c4..1a5006329f6f 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -1510,13 +1510,12 @@ static int write_inode(struct inode *inode, struct writeback_control *wbc)
>   * Wait for writeback on an inode to complete. Called with i_lock held.
>   * Caller must make sure inode cannot go away when we drop i_lock.
>   */
> -static void __inode_wait_for_writeback(struct inode *inode)
> -	__releases(inode->i_lock)
> -	__acquires(inode->i_lock)
> +void inode_wait_for_writeback(struct inode *inode)
>  {
>  	DEFINE_WAIT_BIT(wq, &inode->i_state, __I_SYNC);
>  	wait_queue_head_t *wqh;
>  
> +	lockdep_assert_held(&inode->i_lock);
>  	wqh = bit_waitqueue(&inode->i_state, __I_SYNC);
>  	while (inode->i_state & I_SYNC) {
>  		spin_unlock(&inode->i_lock);
> @@ -1526,16 +1525,6 @@ static void __inode_wait_for_writeback(struct inode *inode)
>  	}
>  }
>  
> -/*
> - * Wait for writeback on an inode to complete. Caller must have inode pinned.
> - */
> -void inode_wait_for_writeback(struct inode *inode)
> -{
> -	spin_lock(&inode->i_lock);
> -	__inode_wait_for_writeback(inode);
> -	spin_unlock(&inode->i_lock);
> -}
> -
>  /*
>   * Sleep until I_SYNC is cleared. This function must be called with i_lock
>   * held and drops it. It is aimed for callers not holding any inode reference
> @@ -1757,7 +1746,7 @@ static int writeback_single_inode(struct inode *inode,
>  		 */
>  		if (wbc->sync_mode != WB_SYNC_ALL)
>  			goto out;
> -		__inode_wait_for_writeback(inode);
> +		inode_wait_for_writeback(inode);
>  	}
>  	WARN_ON(inode->i_state & I_SYNC);
>  	/*
> diff --git a/fs/inode.c b/fs/inode.c
> index 73183a499b1c..d48d29d39cd2 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -582,7 +582,7 @@ static void inode_unpin_lru_isolating(struct inode *inode)
>  
>  static void inode_wait_for_lru_isolating(struct inode *inode)
>  {
> -	spin_lock(&inode->i_lock);
> +	lockdep_assert_held(&inode->i_lock);
>  	if (inode->i_state & I_LRU_ISOLATING) {
>  		DEFINE_WAIT_BIT(wq, &inode->i_state, __I_LRU_ISOLATING);
>  		wait_queue_head_t *wqh;
> @@ -593,7 +593,6 @@ static void inode_wait_for_lru_isolating(struct inode *inode)
>  		spin_lock(&inode->i_lock);
>  		WARN_ON(inode->i_state & I_LRU_ISOLATING);
>  	}
> -	spin_unlock(&inode->i_lock);
>  }
>  
>  /**
> @@ -765,6 +764,7 @@ static void evict(struct inode *inode)
>  
>  	inode_sb_list_del(inode);
>  
> +	spin_lock(&inode->i_lock);
>  	inode_wait_for_lru_isolating(inode);
>  
>  	/*
> @@ -774,6 +774,7 @@ static void evict(struct inode *inode)
>  	 * the inode.  We just have to wait for running writeback to finish.
>  	 */
>  	inode_wait_for_writeback(inode);
> +	spin_unlock(&inode->i_lock);
>  
>  	if (op->evict_inode) {
>  		op->evict_inode(inode);
> -- 
> 2.43.0
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR