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From: Jan Kara <jack@suse.cz>
To: Eric Biggers <ebiggers@kernel.org>
Cc: linux-fsdevel@vger.kernel.org, linux-xfs@vger.kernel.org,
	linux-ext4@vger.kernel.org,
	linux-f2fs-devel@lists.sourceforge.net,
	Theodore Ts'o <tytso@mit.edu>, Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH v2 09/12] fs: improve comments for writeback_single_inode()
Date: Mon, 11 Jan 2021 16:05:23 +0100	[thread overview]
Message-ID: <20210111150523.GH18475@quack2.suse.cz> (raw)
In-Reply-To: <20210109075903.208222-10-ebiggers@kernel.org>

On Fri 08-01-21 23:59:00, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.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 __writeback_single_inode() are outdated or not very helpful, especially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@google.com>

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

Reviewed-by: Jan Kara <jack@suse.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@suse.com>
SUSE Labs, CR

WARNING: multiple messages have this Message-ID
From: Jan Kara <jack@suse.cz>
To: Eric Biggers <ebiggers@kernel.org>
Cc: Theodore Ts'o <tytso@mit.edu>,
	linux-f2fs-devel@lists.sourceforge.net,
	linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-ext4@vger.kernel.org, Christoph Hellwig <hch@lst.de>
Subject: Re: [f2fs-dev] [PATCH v2 09/12] fs: improve comments for writeback_single_inode()
Date: Mon, 11 Jan 2021 16:05:23 +0100	[thread overview]
Message-ID: <20210111150523.GH18475@quack2.suse.cz> (raw)
In-Reply-To: <20210109075903.208222-10-ebiggers@kernel.org>

On Fri 08-01-21 23:59:00, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.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@google.com>

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

Reviewed-by: Jan Kara <jack@suse.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@suse.com>
SUSE Labs, CR


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  parent reply	other threads:[~2021-01-11 15:06 UTC|newest]

Thread overview: 82+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-09  7:58 [PATCH v2 00/12] lazytime fix and cleanups Eric Biggers
2021-01-09  7:58 ` [f2fs-dev] " Eric Biggers
2021-01-09  7:58 ` [PATCH v2 01/12] fs: fix lazytime expiration handling in __writeback_single_inode() Eric Biggers
2021-01-09  7:58   ` [f2fs-dev] " Eric Biggers
2021-01-11 10:48   ` Christoph Hellwig
2021-01-11 10:48     ` [f2fs-dev] " Christoph Hellwig
2021-01-11 14:46   ` Jan Kara
2021-01-11 14:46     ` [f2fs-dev] " Jan Kara
2021-01-09  7:58 ` [PATCH v2 02/12] fs: correctly document the inode dirty flags Eric Biggers
2021-01-09  7:58   ` [f2fs-dev] " Eric Biggers
2021-01-11 14:48   ` Jan Kara
2021-01-11 14:48     ` [f2fs-dev] " Jan Kara
2021-01-09  7:58 ` [PATCH v2 03/12] fs: only specify I_DIRTY_TIME when needed in generic_update_time() Eric Biggers
2021-01-09  7:58   ` [f2fs-dev] " Eric Biggers
2021-01-11 14:50   ` Jan Kara
2021-01-11 14:50     ` [f2fs-dev] " Jan Kara
2021-01-09  7:58 ` [PATCH v2 04/12] fat: only specify I_DIRTY_TIME when needed in fat_update_time() Eric Biggers
2021-01-09  7:58   ` [f2fs-dev] " Eric Biggers
2021-01-11 10:52   ` Christoph Hellwig
2021-01-11 10:52     ` [f2fs-dev] " Christoph Hellwig
2021-01-11 19:50     ` Eric Biggers
2021-01-11 19:50       ` [f2fs-dev] " Eric Biggers
2021-01-12  5:21       ` Dave Chinner
2021-01-12  5:21         ` [f2fs-dev] " Dave Chinner
2021-01-12 13:23       ` Christoph Hellwig
2021-01-12 13:23         ` [f2fs-dev] " Christoph Hellwig
2021-01-11 14:52   ` Jan Kara
2021-01-11 14:52     ` [f2fs-dev] " Jan Kara
2021-01-09  7:58 ` [PATCH v2 05/12] fs: don't call ->dirty_inode for lazytime timestamp updates Eric Biggers
2021-01-09  7:58   ` [f2fs-dev] " Eric Biggers
2021-01-11 14:54   ` Jan Kara
2021-01-11 14:54     ` [f2fs-dev] " Jan Kara
2021-01-09  7:58 ` [PATCH v2 06/12] fs: pass only I_DIRTY_INODE flags to ->dirty_inode Eric Biggers
2021-01-09  7:58   ` [f2fs-dev] " Eric Biggers
2021-01-11 14:56   ` Jan Kara
2021-01-11 14:56     ` [f2fs-dev] " Jan Kara
2021-01-09  7:58 ` [PATCH v2 07/12] fs: clean up __mark_inode_dirty() a bit Eric Biggers
2021-01-09  7:58   ` [f2fs-dev] " Eric Biggers
2021-01-11 14:59   ` Jan Kara
2021-01-11 14:59     ` [f2fs-dev] " Jan Kara
2021-01-09  7:58 ` [PATCH v2 08/12] fs: drop redundant check from __writeback_single_inode() Eric Biggers
2021-01-09  7:58   ` [f2fs-dev] " Eric Biggers
2021-01-11 10:52   ` Christoph Hellwig
2021-01-11 10:52     ` [f2fs-dev] " Christoph Hellwig
2021-01-11 15:00   ` Jan Kara
2021-01-11 15:00     ` [f2fs-dev] " Jan Kara
2021-01-09  7:59 ` [PATCH v2 09/12] fs: improve comments for writeback_single_inode() Eric Biggers
2021-01-09  7:59   ` [f2fs-dev] " Eric Biggers
2021-01-11 10:53   ` Christoph Hellwig
2021-01-11 10:53     ` [f2fs-dev] " Christoph Hellwig
2021-01-11 15:05   ` Jan Kara [this message]
2021-01-11 15:05     ` Jan Kara
2021-01-09  7:59 ` [PATCH v2 10/12] gfs2: don't worry about I_DIRTY_TIME in gfs2_fsync() Eric Biggers
2021-01-09  7:59   ` [f2fs-dev] " Eric Biggers
2021-01-11 15:06   ` Jan Kara
2021-01-11 15:06     ` [f2fs-dev] " Jan Kara
2021-01-09  7:59 ` [PATCH v2 11/12] ext4: simplify i_state checks in __ext4_update_other_inode_time() Eric Biggers
2021-01-09  7:59   ` [f2fs-dev] " Eric Biggers
2021-01-11 10:53   ` Christoph Hellwig
2021-01-11 10:53     ` [f2fs-dev] " Christoph Hellwig
2021-01-11 20:23     ` Eric Biggers
2021-01-11 20:23       ` [f2fs-dev] " Eric Biggers
2021-01-12 13:25       ` Christoph Hellwig
2021-01-12 13:25         ` [f2fs-dev] " Christoph Hellwig
2021-02-03  5:16         ` Theodore Ts'o
2021-02-03  5:16           ` [f2fs-dev] " Theodore Ts'o
2021-01-11 15:11   ` Jan Kara
2021-01-11 15:11     ` [f2fs-dev] " Jan Kara
2021-01-09  7:59 ` [PATCH v2 12/12] xfs: remove a stale comment from xfs_file_aio_write_checks() Eric Biggers
2021-01-09  7:59   ` [f2fs-dev] " Eric Biggers
2021-01-12 17:31   ` Darrick J. Wong
2021-01-12 17:31     ` [f2fs-dev] " Darrick J. Wong
2021-01-11 15:15 ` [PATCH v2 00/12] lazytime fix and cleanups Jan Kara
2021-01-11 15:15   ` [f2fs-dev] " Jan Kara
2021-01-11 20:44   ` Eric Biggers
2021-01-11 20:44     ` [f2fs-dev] " Eric Biggers
2021-02-03  5:11     ` Theodore Ts'o
2021-02-03  5:11       ` [f2fs-dev] " Theodore Ts'o
2021-02-03  5:22       ` Eric Biggers
2021-02-03  5:22         ` [f2fs-dev] " Eric Biggers
2021-02-03 15:49         ` Theodore Ts'o
2021-02-03 15:49           ` [f2fs-dev] " Theodore Ts'o

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