[PATCH v3 09/11] fs: improve comments for writeback_single_inode()

Eric Biggers
2021-01-12T19:05:33Z

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__writeback_single_inode() are outdated or not very helpful, especially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.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
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