Date:   Mon, 11 Jan 2021 16:05:23 +0100
From:   Jan Kara <jack@...e.cz>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
        Theodore Ts'o <tytso@....edu>, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v2 09/12] fs: improve comments for
 writeback_single_inode()

On Fri 08-01-21 23:59:00, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@...gle.com>
> 
> Some comments for writeback_single_inode() and
> __writeback_single_inode() are outdated or not very helpful, especially
> with regards to writeback list handling.  Update them.
> 
> Signed-off-by: Eric Biggers <ebiggers@...gle.com>

Yeah, looks more comprehensible :). Thanks for the cleanup. Feel free to
add:

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

								Honza

> ---
>  fs/fs-writeback.c | 57 +++++++++++++++++++++++++++--------------------
>  1 file changed, 33 insertions(+), 24 deletions(-)
> 
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index cee1df6e3bd43..e91980f493884 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -1442,9 +1442,15 @@ static void requeue_inode(struct inode *inode, struct bdi_writeback *wb,
>  }
>  
>  /*
> - * Write out an inode and its dirty pages. Do not update the writeback list
> - * linkage. That is left to the caller. The caller is also responsible for
> - * setting I_SYNC flag and calling inode_sync_complete() to clear it.
> + * Write out an inode and its dirty pages (or some of its dirty pages, depending
> + * on @wbc->nr_to_write), and clear the relevant dirty flags from i_state.
> + *
> + * This doesn't remove the inode from the writeback list it is on, except
> + * potentially to move it from b_dirty_time to b_dirty due to timestamp
> + * expiration.  The caller is otherwise responsible for writeback list handling.
> + *
> + * The caller is also responsible for setting the I_SYNC flag beforehand and
> + * calling inode_sync_complete() to clear it afterwards.
>   */
>  static int
>  __writeback_single_inode(struct inode *inode, struct writeback_control *wbc)
> @@ -1487,9 +1493,10 @@ __writeback_single_inode(struct inode *inode, struct writeback_control *wbc)
>  	}
>  
>  	/*
> -	 * Some filesystems may redirty the inode during the writeback
> -	 * due to delalloc, clear dirty metadata flags right before
> -	 * write_inode()
> +	 * Get and clear the dirty flags from i_state.  This needs to be done
> +	 * after calling writepages because some filesystems may redirty the
> +	 * inode during writepages due to delalloc.  It also needs to be done
> +	 * after handling timestamp expiration, as that may dirty the inode too.
>  	 */
>  	spin_lock(&inode->i_lock);
>  	dirty = inode->i_state & I_DIRTY;
> @@ -1524,12 +1531,13 @@ __writeback_single_inode(struct inode *inode, struct writeback_control *wbc)
>  }
>  
>  /*
> - * Write out an inode's dirty pages. Either the caller has an active reference
> - * on the inode or the inode has I_WILL_FREE set.
> + * Write out an inode's dirty data and metadata on-demand, i.e. separately from
> + * the regular batched writeback done by the flusher threads in
> + * writeback_sb_inodes().  @wbc controls various aspects of the write, such as
> + * whether it is a data-integrity sync (%WB_SYNC_ALL) or not (%WB_SYNC_NONE).
>   *
> - * This function is designed to be called for writing back one inode which
> - * we go e.g. from filesystem. Flusher thread uses __writeback_single_inode()
> - * and does more profound writeback list handling in writeback_sb_inodes().
> + * To prevent the inode from going away, either the caller must have a reference
> + * to the inode, or the inode must have I_WILL_FREE or I_FREEING set.
>   */
>  static int writeback_single_inode(struct inode *inode,
>  				  struct writeback_control *wbc)
> @@ -1544,23 +1552,23 @@ static int writeback_single_inode(struct inode *inode,
>  		WARN_ON(inode->i_state & I_WILL_FREE);
>  
>  	if (inode->i_state & I_SYNC) {
> -		if (wbc->sync_mode != WB_SYNC_ALL)
> -			goto out;
>  		/*
> -		 * It's a data-integrity sync. We must wait. Since callers hold
> -		 * inode reference or inode has I_WILL_FREE set, it cannot go
> -		 * away under us.
> +		 * Writeback is already running on the inode.  For WB_SYNC_NONE,
> +		 * that's enough and we can just return.  For WB_SYNC_ALL, we
> +		 * must wait for the existing writeback to complete, then do
> +		 * writeback again if there's anything left.
>  		 */
> +		if (wbc->sync_mode != WB_SYNC_ALL)
> +			goto out;
>  		__inode_wait_for_writeback(inode);
>  	}
>  	WARN_ON(inode->i_state & I_SYNC);
>  	/*
> -	 * Skip inode if it is clean and we have no outstanding writeback in
> -	 * WB_SYNC_ALL mode. We don't want to mess with writeback lists in this
> -	 * function since flusher thread may be doing for example sync in
> -	 * parallel and if we move the inode, it could get skipped. So here we
> -	 * make sure inode is on some writeback list and leave it there unless
> -	 * we have completely cleaned the inode.
> +	 * If the inode is already fully clean, then there's nothing to do.
> +	 *
> +	 * For data-integrity syncs we also need to check whether any pages are
> +	 * still under writeback, e.g. due to prior WB_SYNC_NONE writeback.  If
> +	 * there are any such pages, we'll need to wait for them.
>  	 */
>  	if (!(inode->i_state & I_DIRTY_ALL) &&
>  	    (wbc->sync_mode != WB_SYNC_ALL ||
> @@ -1576,8 +1584,9 @@ static int writeback_single_inode(struct inode *inode,
>  	wb = inode_to_wb_and_lock_list(inode);
>  	spin_lock(&inode->i_lock);
>  	/*
> -	 * If inode is clean, remove it from writeback lists. Otherwise don't
> -	 * touch it. See comment above for explanation.
> +	 * If the inode is now fully clean, then it can be safely removed from
> +	 * its writeback list (if any).  Otherwise the flusher threads are
> +	 * responsible for the writeback lists.
>  	 */
>  	if (!(inode->i_state & I_DIRTY_ALL))
>  		inode_io_list_del_locked(inode, wb);
> -- 
> 2.30.0
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR

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