[PATCH 12/24] xfs: pin inode backing buffer to the inode log item

[Dave Chinner <david@fromorbit.com>]

From: Dave Chinner <dchinner@redhat.com>

When we dirty an inode, we are going to have to write it disk at
some point in the near future. This requires the inode cluster
backing buffer to be present in memory. Unfortunately, under severe
memory pressure we can reclaim the inode backing buffer while the
inode is dirty in memory, resulting in stalling the AIL pushing
because it has to do a read-modify-write cycle on the cluster
buffer.

When we have no memory available, the read of the cluster buffer
blocks the AIL pushing process, and this causes all sorts of issues
for memory reclaim as it requires inode writeback to make forwards
progress. Allocating a cluster buffer causes more memory pressure,
and results in more cluster buffers to be reclaimed, resulting in
more RMW cycles to be done in the AIL context and everything then
backs up on AIL progress. Only the synchronous inode cluster
writeback in the the inode reclaim code provides some level of
forwards progress guarantees that prevent OOM-killer rampages in
this situation.

Fix this by pinning the inode backing buffer to the inode log item
when the inode is first dirtied (i.e. in xfs_trans_log_inode()).
This may mean the first modification of an inode that has been held
in cache for a long time may block on a cluster buffer read, but
we can do that in transaction context and block safely until the
buffer has been allocated and read.

Once we have the cluster buffer, the inode log item takes a
reference to it, pinning it in memory, and attaches it to the log
item for future reference. This means we can always grab the cluster
buffer from the inode log item when we need it.

When the inode is finally cleaned and removed from the AIL, we can
drop the reference the inode log item holds on the cluster buffer.
Once all inodes on the cluster buffer are clean, the cluster buffer
will be unpinned and it will be available for memory reclaim to
reclaim again.

This avoids the issues with needing to do RMW cycles in the AIL
pushing context, and hence allows complete non-blocking inode
flushing to be performed by the AIL pushing context.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
---
 fs/xfs/libxfs/xfs_inode_buf.c   |  3 +-
 fs/xfs/libxfs/xfs_trans_inode.c | 71 +++++++++++++++++++++++++--------
 fs/xfs/xfs_inode_item.c         | 63 ++++++++++++++++++++++++-----
 fs/xfs/xfs_trans_priv.h         | 12 +-----
 4 files changed, 112 insertions(+), 37 deletions(-)

diff --git a/fs/xfs/libxfs/xfs_inode_buf.c b/fs/xfs/libxfs/xfs_inode_buf.c
index 6f84ea85fdd83..1af97235785c8 100644
--- a/fs/xfs/libxfs/xfs_inode_buf.c
+++ b/fs/xfs/libxfs/xfs_inode_buf.c
@@ -176,7 +176,8 @@ xfs_imap_to_bp(
 	}
 
 	*bpp = bp;
-	*dipp = xfs_buf_offset(bp, imap->im_boffset);
+	if (dipp)
+		*dipp = xfs_buf_offset(bp, imap->im_boffset);
 	return 0;
 }
 
diff --git a/fs/xfs/libxfs/xfs_trans_inode.c b/fs/xfs/libxfs/xfs_trans_inode.c
index 510b996008221..e130eb2994156 100644
--- a/fs/xfs/libxfs/xfs_trans_inode.c
+++ b/fs/xfs/libxfs/xfs_trans_inode.c
@@ -8,6 +8,8 @@
 #include "xfs_shared.h"
 #include "xfs_format.h"
 #include "xfs_log_format.h"
+#include "xfs_trans_resv.h"
+#include "xfs_mount.h"
 #include "xfs_inode.h"
 #include "xfs_trans.h"
 #include "xfs_trans_priv.h"
@@ -71,13 +73,19 @@ xfs_trans_ichgtime(
 }
 
 /*
- * This is called to mark the fields indicated in fieldmask as needing
- * to be logged when the transaction is committed.  The inode must
- * already be associated with the given transaction.
+ * This is called to mark the fields indicated in fieldmask as needing to be
+ * logged when the transaction is committed.  The inode must already be
+ * associated with the given transaction.
  *
- * The values for fieldmask are defined in xfs_inode_item.h.  We always
- * log all of the core inode if any of it has changed, and we always log
- * all of the inline data/extents/b-tree root if any of them has changed.
+ * The values for fieldmask are defined in xfs_inode_item.h.  We always log all
+ * of the core inode if any of it has changed, and we always log all of the
+ * inline data/extents/b-tree root if any of them has changed.
+ *
+ * Grab and pin the cluster buffer associated with this inode to avoid RMW
+ * cycles at inode writeback time. Avoid the need to add error handling to every
+ * xfs_trans_log_inode() call by shutting down on read error.  This will cause
+ * transactions to fail and everything to error out, just like if we return a
+ * read error in a dirty transaction and cancel it.
  */
 void
 xfs_trans_log_inode(
@@ -122,21 +130,52 @@ xfs_trans_log_inode(
 	}
 
 	/*
-	 * Record the specific change for fdatasync optimisation. This
-	 * allows fdatasync to skip log forces for inodes that are only
-	 * timestamp dirty. We do this before the change count so that
-	 * the core being logged in this case does not impact on fdatasync
-	 * behaviour.
+	 * Record the specific change for fdatasync optimisation. This allows
+	 * fdatasync to skip log forces for inodes that are only timestamp
+	 * dirty. We do this before the change count so that the core being
+	 * logged in this case does not impact on fdatasync behaviour.
 	 */
 	spin_lock(&iip->ili_lock);
 	iip->ili_fsync_fields |= flags;
 
+	if (!iip->ili_item.li_buf) {
+		struct xfs_buf	*bp;
+		int		error;
+
+		/*
+		 * We hold the ILOCK here, so this inode is not going to be
+		 * flushed while we are here. Further, because there is no
+		 * buffer attached to the item, we know that there is no IO in
+		 * progress, so nothing will clear the ili_fields while we read
+		 * in the buffer. Hence we can safely drop the spin lock and
+		 * read the buffer knowing that the state will not change from
+		 * here.
+		 */
+		spin_unlock(&iip->ili_lock);
+		error = xfs_imap_to_bp(ip->i_mount, tp, &ip->i_imap, NULL,
+					&bp, 0);
+		if (error) {
+			xfs_force_shutdown(ip->i_mount, SHUTDOWN_META_IO_ERROR);
+			return;
+		}
+
+		/*
+		 * We need an explicit buffer reference for the log item, We
+		 * don't want the buffer attached to the transaction, so we have
+		 * to release the transaction reference we just gained.
+		 */
+		xfs_buf_hold(bp);
+		xfs_trans_brelse(tp, bp);
+
+		spin_lock(&iip->ili_lock);
+		iip->ili_item.li_buf = bp;
+	}
+
 	/*
-	 * Always OR in the bits from the ili_last_fields field.
-	 * This is to coordinate with the xfs_iflush() and xfs_iflush_done()
-	 * routines in the eventual clearing of the ili_fields bits.
-	 * See the big comment in xfs_iflush() for an explanation of
-	 * this coordination mechanism.
+	 * Always OR in the bits from the ili_last_fields field.  This is to
+	 * coordinate with the xfs_iflush() and xfs_iflush_done() routines in
+	 * the eventual clearing of the ili_fields bits.  See the big comment in
+	 * xfs_iflush() for an explanation of this coordination mechanism.
 	 */
 	flags |= iip->ili_last_fields | iversion_flags;
 	iip->ili_fields |= flags;
diff --git a/fs/xfs/xfs_inode_item.c b/fs/xfs/xfs_inode_item.c
index 7049f2ae8d186..86173a52526fe 100644
--- a/fs/xfs/xfs_inode_item.c
+++ b/fs/xfs/xfs_inode_item.c
@@ -130,6 +130,8 @@ xfs_inode_item_size(
 	xfs_inode_item_data_fork_size(iip, nvecs, nbytes);
 	if (XFS_IFORK_Q(ip))
 		xfs_inode_item_attr_fork_size(iip, nvecs, nbytes);
+
+	ASSERT(iip->ili_item.li_buf);
 }
 
 STATIC void
@@ -439,6 +441,7 @@ xfs_inode_item_pin(
 	struct xfs_inode	*ip = INODE_ITEM(lip)->ili_inode;
 
 	ASSERT(xfs_isilocked(ip, XFS_ILOCK_EXCL));
+	ASSERT(lip->li_buf);
 
 	trace_xfs_inode_pin(ip, _RET_IP_);
 	atomic_inc(&ip->i_pincount);
@@ -450,6 +453,12 @@ xfs_inode_item_pin(
  * item which was previously pinned with a call to xfs_inode_item_pin().
  *
  * Also wake up anyone in xfs_iunpin_wait() if the count goes to 0.
+ *
+ * Note that unpin can race with inode cluster buffer freeing marking the buffer
+ * stale. In that case, flush completions are run from the buffer unpin call,
+ * which may happen before the inode is unpinned. If we lose the race, there
+ * will be no buffer attached to the log item, but the inode will be marked
+ * XFS_ISTALE.
  */
 STATIC void
 xfs_inode_item_unpin(
@@ -459,6 +468,7 @@ xfs_inode_item_unpin(
 	struct xfs_inode	*ip = INODE_ITEM(lip)->ili_inode;
 
 	trace_xfs_inode_unpin(ip, _RET_IP_);
+	ASSERT(lip->li_buf || xfs_iflags_test(ip, XFS_ISTALE));
 	ASSERT(atomic_read(&ip->i_pincount) > 0);
 	if (atomic_dec_and_test(&ip->i_pincount))
 		wake_up_bit(&ip->i_flags, __XFS_IPINNED_BIT);
@@ -647,10 +657,15 @@ xfs_inode_item_init(
  */
 void
 xfs_inode_item_destroy(
-	xfs_inode_t	*ip)
+	struct xfs_inode	*ip)
 {
-	kmem_free(ip->i_itemp->ili_item.li_lv_shadow);
-	kmem_cache_free(xfs_ili_zone, ip->i_itemp);
+	struct xfs_inode_log_item *iip = ip->i_itemp;
+
+	ASSERT(iip->ili_item.li_buf == NULL);
+
+	ip->i_itemp = NULL;
+	kmem_free(iip->ili_item.li_lv_shadow);
+	kmem_cache_free(xfs_ili_zone, iip);
 }
 
 
@@ -665,6 +680,13 @@ xfs_inode_item_destroy(
  * list for other inodes that will run this function. We remove them from the
  * buffer list so we can process all the inode IO completions in one AIL lock
  * traversal.
+ *
+ * Note: Now that we attach the log item to the buffer when we first log the
+ * inode in memory, we can have unflushed inodes on the buffer list here. These
+ * inodes will have a zero ili_last_fields, so skip over them here. We do
+ * this check -after- we've checked for stale inodes, because we're guaranteed
+ * to have XFS_ISTALE set in the case that dirty inodes are in the CIL and have
+ * not yet had their dirtying transactions committed to disk.
  */
 void
 xfs_iflush_done(
@@ -688,14 +710,16 @@ xfs_iflush_done(
 			continue;
 		}
 
+		if (!iip->ili_last_fields)
+			continue;
+
 		list_move_tail(&lip->li_bio_list, &tmp);
 
 		/* Do an unlocked check for needing the AIL lock. */
-		if (lip->li_lsn == iip->ili_flush_lsn ||
+		if (iip->ili_flush_lsn == lip->li_lsn ||
 		    test_bit(XFS_LI_FAILED, &lip->li_flags))
 			need_ail++;
 	}
-	ASSERT(list_empty(&bp->b_li_list));
 
 	/*
 	 * We only want to pull the item from the AIL if it is actually there
@@ -708,7 +732,8 @@ xfs_iflush_done(
 		/* this is an opencoded batch version of xfs_trans_ail_delete */
 		spin_lock(&ailp->ail_lock);
 		list_for_each_entry(lip, &tmp, li_bio_list) {
-			if (lip->li_lsn == INODE_ITEM(lip)->ili_flush_lsn) {
+			iip = INODE_ITEM(lip);
+			if (iip->ili_flush_lsn == lip->li_lsn) {
 				xfs_lsn_t lsn = xfs_ail_delete_one(ailp, lip);
 				if (!tail_lsn && lsn)
 					tail_lsn = lsn;
@@ -725,14 +750,29 @@ xfs_iflush_done(
 	 * them is safely on disk.
 	 */
 	list_for_each_entry_safe(lip, n, &tmp, li_bio_list) {
+		bool	drop_buffer = false;
+
 		list_del_init(&lip->li_bio_list);
 		iip = INODE_ITEM(lip);
 
 		spin_lock(&iip->ili_lock);
 		iip->ili_last_fields = 0;
-		spin_unlock(&iip->ili_lock);
+		iip->ili_flush_lsn = 0;
 
+		/*
+		 * Remove the reference to the cluster buffer if the inode is
+		 * clean in memory. Drop the buffer reference once we've dropped
+		 * the locks we hold.
+		 */
+		ASSERT(iip->ili_item.li_buf == bp);
+		if (!iip->ili_fields) {
+			iip->ili_item.li_buf = NULL;
+			drop_buffer = true;
+		}
+		spin_unlock(&iip->ili_lock);
 		xfs_ifunlock(iip->ili_inode);
+		if (drop_buffer)
+			xfs_buf_rele(bp);
 	}
 }
 
@@ -747,6 +787,7 @@ xfs_iflush_abort(
 	struct xfs_inode		*ip)
 {
 	struct xfs_inode_log_item	*iip = ip->i_itemp;
+	struct xfs_buf		*bp = NULL;
 
 	if (iip) {
 		xfs_trans_ail_delete(&iip->ili_item, 0);
@@ -758,12 +799,14 @@ xfs_iflush_abort(
 		iip->ili_last_fields = 0;
 		iip->ili_fields = 0;
 		iip->ili_fsync_fields = 0;
+		iip->ili_flush_lsn = 0;
+		bp = iip->ili_item.li_buf;
+		iip->ili_item.li_buf = NULL;
 		spin_unlock(&iip->ili_lock);
 	}
-	/*
-	 * Release the inode's flush lock since we're done with it.
-	 */
 	xfs_ifunlock(ip);
+	if (bp)
+		xfs_buf_rele(bp);
 }
 
 /*
diff --git a/fs/xfs/xfs_trans_priv.h b/fs/xfs/xfs_trans_priv.h
index 3004aeac91102..21ffc6dfcd13e 100644
--- a/fs/xfs/xfs_trans_priv.h
+++ b/fs/xfs/xfs_trans_priv.h
@@ -143,15 +143,10 @@ static inline void
 xfs_clear_li_failed(
 	struct xfs_log_item	*lip)
 {
-	struct xfs_buf	*bp = lip->li_buf;
-
 	ASSERT(test_bit(XFS_LI_IN_AIL, &lip->li_flags));
 	lockdep_assert_held(&lip->li_ailp->ail_lock);
 
-	if (test_and_clear_bit(XFS_LI_FAILED, &lip->li_flags)) {
-		lip->li_buf = NULL;
-		xfs_buf_rele(bp);
-	}
+	clear_bit(XFS_LI_FAILED, &lip->li_flags);
 }
 
 static inline void
@@ -161,10 +156,7 @@ xfs_set_li_failed(
 {
 	lockdep_assert_held(&lip->li_ailp->ail_lock);
 
-	if (!test_and_set_bit(XFS_LI_FAILED, &lip->li_flags)) {
-		xfs_buf_hold(bp);
-		lip->li_buf = bp;
-	}
+	set_bit(XFS_LI_FAILED, &lip->li_flags);
 }
 
 #endif	/* __XFS_TRANS_PRIV_H__ */
-- 
2.26.2.761.g0e0b3e54be