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From: Eric Biggers <ebiggers@kernel.org>
To: linux-fsdevel@vger.kernel.org
Cc: 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: [PATCH v2 09/12] fs: improve comments for writeback_single_inode()
Date: Fri,  8 Jan 2021 23:59:00 -0800	[thread overview]
Message-ID: <20210109075903.208222-10-ebiggers@kernel.org> (raw)
In-Reply-To: <20210109075903.208222-1-ebiggers@kernel.org>

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__writeback_single_inode() are outdated or not very helpful, especially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>
---
 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


WARNING: multiple messages have this Message-ID
From: Eric Biggers <ebiggers@kernel.org>
To: linux-fsdevel@vger.kernel.org
Cc: linux-xfs@vger.kernel.org, linux-ext4@vger.kernel.org,
	Theodore Ts'o <tytso@mit.edu>, Christoph Hellwig <hch@lst.de>,
	linux-f2fs-devel@lists.sourceforge.net
Subject: [f2fs-dev] [PATCH v2 09/12] fs: improve comments for writeback_single_inode()
Date: Fri,  8 Jan 2021 23:59:00 -0800	[thread overview]
Message-ID: <20210109075903.208222-10-ebiggers@kernel.org> (raw)
In-Reply-To: <20210109075903.208222-1-ebiggers@kernel.org>

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>
---
 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



_______________________________________________
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  parent reply	other threads:[~2021-01-09  8:01 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 ` Eric Biggers [this message]
2021-01-09  7:59   ` [f2fs-dev] [PATCH v2 09/12] fs: improve comments for writeback_single_inode() 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
2021-01-11 15:05     ` [f2fs-dev] " 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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