xfs: allocate log vector buffers outside CIL context lock

One of the problems we currently have with delayed logging is that
under serious memory pressure we can deadlock memory reclaim. THis
occurs when memory reclaim (such as run by kswapd) is reclaiming XFS
inodes and issues a log force to unpin inodes that are dirty in the
CIL.

The CIL is pushed, but this will only occur once it gets the CIL
context lock to ensure that all committing transactions are complete
and no new transactions start being committed to the CIL while the
push switches to a new context.

The deadlock occurs when the CIL context lock is held by a
committing process that is doing memory allocation for log vector
buffers, and that allocation is then blocked on memory reclaim
making progress. Memory reclaim, however, is blocked waiting for
a log force to make progress, and so we effectively deadlock at this
point.

To solve this problem, we have to move the CIL log vector buffer
allocation outside of the context lock so that memory reclaim can
always make progress when it needs to force the log. The problem
with doing this is that a CIL push can take place while we are
determining if we need to allocate a new log vector buffer for
an item and hence the current log vector may go away without
warning. That means we canot rely on the existing log vector being
present when we finally grab the context lock and so we must have a
replacement buffer ready to go at all times.

To ensure this, introduce a "shadow log vector" buffer that is
always guaranteed to be present when we gain the CIL context lock
and format the item. This shadow buffer may or may not be used
during the formatting, but if the log item does not have an existing
log vector buffer or that buffer is too small for the new
modifications, we swap it for the new shadow buffer and format
the modifications into that new log vector buffer.

The result of this is that for any object we modify more than once
in a given CIL checkpoint, we double the memory required
to track dirty regions in the log. For single modifications then
we consume the shadow log vectorwe allocate on commit, and that gets
consumed by the checkpoint. However, if we make multiple
modifications, then the second transaction commit will allocate a
shadow log vector and hence we will end up with double the memory
usage as only one of the log vectors is consumed by the CIL
checkpoint. The remaining shadow vector will be freed when th elog
item is freed.

This can probably be optimised in future - access to the shadow log
vector is serialised by the object lock (as opposited to the active
log vector, which is controlled by the CIL context lock) and so we
can probably free shadow log vector from some objects when the log
item is marked clean on removal from the AIL.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

fs/xfs/xfs_buf_item.c

@@ -949,6 +949,7 @@ xfs_buf_item_free(
 	xfs_buf_log_item_t	*bip)
 {
 	xfs_buf_item_free_format(bip);
+	kmem_free(bip->bli_item.li_lv_shadow);
 	kmem_zone_free(xfs_buf_item_zone, bip);
 }
 

fs/xfs/xfs_dquot.c

@@ -74,6 +74,7 @@ xfs_qm_dqdestroy(
 {
 	ASSERT(list_empty(&dqp->q_lru));
 
+	kmem_free(dqp->q_logitem.qli_item.li_lv_shadow);
 	mutex_destroy(&dqp->q_qlock);
 
 	XFS_STATS_DEC(dqp->q_mount, xs_qm_dquot);

fs/xfs/xfs_dquot_item.c

@@ -370,6 +370,8 @@ xfs_qm_qoffend_logitem_committed(
 	spin_lock(&ailp->xa_lock);
 	xfs_trans_ail_delete(ailp, &qfs->qql_item, SHUTDOWN_LOG_IO_ERROR);
 
+	kmem_free(qfs->qql_item.li_lv_shadow);
+	kmem_free(lip->li_lv_shadow);
 	kmem_free(qfs);
 	kmem_free(qfe);
 	return (xfs_lsn_t)-1;

fs/xfs/xfs_extfree_item.c

@@ -40,6 +40,7 @@ void
 xfs_efi_item_free(
 	struct xfs_efi_log_item	*efip)
 {
+	kmem_free(efip->efi_item.li_lv_shadow);
 	if (efip->efi_format.efi_nextents > XFS_EFI_MAX_FAST_EXTENTS)
 		kmem_free(efip);
 	else
@@ -300,6 +301,7 @@ static inline struct xfs_efd_log_item *EFD_ITEM(struct xfs_log_item *lip)
 STATIC void
 xfs_efd_item_free(struct xfs_efd_log_item *efdp)
 {
+	kmem_free(efdp->efd_item.li_lv_shadow);
 	if (efdp->efd_format.efd_nextents > XFS_EFD_MAX_FAST_EXTENTS)
 		kmem_free(efdp);
 	else

fs/xfs/xfs_inode_item.c

@@ -651,6 +651,7 @@ void
 xfs_inode_item_destroy(
 	xfs_inode_t	*ip)
 {
+	kmem_free(ip->i_itemp->ili_item.li_lv_shadow);
 	kmem_zone_free(xfs_ili_zone, ip->i_itemp);
 }
 

fs/xfs/xfs_log_cil.c

@@ -78,6 +78,157 @@ xlog_cil_init_post_recovery(
 	log->l_cilp->xc_ctx->sequence = 1;
 }
 
+static inline int
+xlog_cil_iovec_space(
+	uint	niovecs)
+{
+	return round_up((sizeof(struct xfs_log_vec) +
+					niovecs * sizeof(struct xfs_log_iovec)),
+			sizeof(uint64_t));
+}
+
+/*
+ * Allocate or pin log vector buffers for CIL insertion.
+ *
+ * The CIL currently uses disposable buffers for copying a snapshot of the
+ * modified items into the log during a push. The biggest problem with this is
+ * the requirement to allocate the disposable buffer during the commit if:
+ *	a) does not exist; or
+ *	b) it is too small
+ *
+ * If we do this allocation within xlog_cil_insert_format_items(), it is done
+ * under the xc_ctx_lock, which means that a CIL push cannot occur during
+ * the memory allocation. This means that we have a potential deadlock situation
+ * under low memory conditions when we have lots of dirty metadata pinned in
+ * the CIL and we need a CIL commit to occur to free memory.
+ *
+ * To avoid this, we need to move the memory allocation outside the
+ * xc_ctx_lock, but because the log vector buffers are disposable, that opens
+ * up a TOCTOU race condition w.r.t. the CIL committing and removing the log
+ * vector buffers between the check and the formatting of the item into the
+ * log vector buffer within the xc_ctx_lock.
+ *
+ * Because the log vector buffer needs to be unchanged during the CIL push
+ * process, we cannot share the buffer between the transaction commit (which
+ * modifies the buffer) and the CIL push context that is writing the changes
+ * into the log. This means skipping preallocation of buffer space is
+ * unreliable, but we most definitely do not want to be allocating and freeing
+ * buffers unnecessarily during commits when overwrites can be done safely.
+ *
+ * The simplest solution to this problem is to allocate a shadow buffer when a
+ * log item is committed for the second time, and then to only use this buffer
+ * if necessary. The buffer can remain attached to the log item until such time
+ * it is needed, and this is the buffer that is reallocated to match the size of
+ * the incoming modification. Then during the formatting of the item we can swap
+ * the active buffer with the new one if we can't reuse the existing buffer. We
+ * don't free the old buffer as it may be reused on the next modification if
+ * it's size is right, otherwise we'll free and reallocate it at that point.
+ *
+ * This function builds a vector for the changes in each log item in the
+ * transaction. It then works out the length of the buffer needed for each log
+ * item, allocates them and attaches the vector to the log item in preparation
+ * for the formatting step which occurs under the xc_ctx_lock.
+ *
+ * While this means the memory footprint goes up, it avoids the repeated
+ * alloc/free pattern that repeated modifications of an item would otherwise
+ * cause, and hence minimises the CPU overhead of such behaviour.
+ */
+static void
+xlog_cil_alloc_shadow_bufs(
+	struct xlog		*log,
+	struct xfs_trans	*tp)
+{
+	struct xfs_log_item_desc *lidp;
+
+	list_for_each_entry(lidp, &tp->t_items, lid_trans) {
+		struct xfs_log_item *lip = lidp->lid_item;
+		struct xfs_log_vec *lv;
+		int	niovecs = 0;
+		int	nbytes = 0;
+		int	buf_size;
+		bool	ordered = false;
+
+		/* Skip items which aren't dirty in this transaction. */
+		if (!(lidp->lid_flags & XFS_LID_DIRTY))
+			continue;
+
+		/* get number of vecs and size of data to be stored */
+		lip->li_ops->iop_size(lip, &niovecs, &nbytes);
+
+		/*
+		 * Ordered items need to be tracked but we do not wish to write
+		 * them. We need a logvec to track the object, but we do not
+		 * need an iovec or buffer to be allocated for copying data.
+		 */
+		if (niovecs == XFS_LOG_VEC_ORDERED) {
+			ordered = true;
+			niovecs = 0;
+			nbytes = 0;
+		}
+
+		/*
+		 * We 64-bit align the length of each iovec so that the start
+		 * of the next one is naturally aligned.  We'll need to
+		 * account for that slack space here. Then round nbytes up
+		 * to 64-bit alignment so that the initial buffer alignment is
+		 * easy to calculate and verify.
+		 */
+		nbytes += niovecs * sizeof(uint64_t);
+		nbytes = round_up(nbytes, sizeof(uint64_t));
+
+		/*
+		 * The data buffer needs to start 64-bit aligned, so round up
+		 * that space to ensure we can align it appropriately and not
+		 * overrun the buffer.
+		 */
+		buf_size = nbytes + xlog_cil_iovec_space(niovecs);
+
+		/*
+		 * if we have no shadow buffer, or it is too small, we need to
+		 * reallocate it.
+		 */
+		if (!lip->li_lv_shadow ||
+		    buf_size > lip->li_lv_shadow->lv_size) {
+
+			/*
+			 * We free and allocate here as a realloc would copy
+			 * unecessary data. We don't use kmem_zalloc() for the
+			 * same reason - we don't need to zero the data area in
+			 * the buffer, only the log vector header and the iovec
+			 * storage.
+			 */
+			kmem_free(lip->li_lv_shadow);
+
+			lv = kmem_alloc(buf_size, KM_SLEEP|KM_NOFS);
+			memset(lv, 0, xlog_cil_iovec_space(niovecs));
+
+			lv->lv_item = lip;
+			lv->lv_size = buf_size;
+			if (ordered)
+				lv->lv_buf_len = XFS_LOG_VEC_ORDERED;
+			else
+				lv->lv_iovecp = (struct xfs_log_iovec *)&lv[1];
+			lip->li_lv_shadow = lv;
+		} else {
+			/* same or smaller, optimise common overwrite case */
+			lv = lip->li_lv_shadow;
+			if (ordered)
+				lv->lv_buf_len = XFS_LOG_VEC_ORDERED;
+			else
+				lv->lv_buf_len = 0;
+			lv->lv_bytes = 0;
+			lv->lv_next = NULL;
+		}
+
+		/* Ensure the lv is set up according to ->iop_size */
+		lv->lv_niovecs = niovecs;
+
+		/* The allocated data region lies beyond the iovec region */
+		lv->lv_buf = (char *)lv + xlog_cil_iovec_space(niovecs);
+	}
+
+}
+
 /*
  * Prepare the log item for insertion into the CIL. Calculate the difference in
  * log space and vectors it will consume, and if it is a new item pin it as
@@ -100,16 +251,19 @@ xfs_cil_prepare_item(
 	/*
 	 * If there is no old LV, this is the first time we've seen the item in
 	 * this CIL context and so we need to pin it. If we are replacing the
-	 * old_lv, then remove the space it accounts for and free it.
+	 * old_lv, then remove the space it accounts for and make it the shadow
+	 * buffer for later freeing. In both cases we are now switching to the
+	 * shadow buffer, so update the the pointer to it appropriately.
 	 */
-	if (!old_lv)
+	if (!old_lv) {
 		lv->lv_item->li_ops->iop_pin(lv->lv_item);
-	else if (old_lv != lv) {
+		lv->lv_item->li_lv_shadow = NULL;
+	} else if (old_lv != lv) {
 		ASSERT(lv->lv_buf_len != XFS_LOG_VEC_ORDERED);
 
 		*diff_len -= old_lv->lv_bytes;
 		*diff_iovecs -= old_lv->lv_niovecs;
-		kmem_free(old_lv);
+		lv->lv_item->li_lv_shadow = old_lv;
 	}
 
 	/* attach new log vector to log item */
@@ -133,11 +287,13 @@ xfs_cil_prepare_item(
  * write it out asynchronously without needing to relock the object that was
  * modified at the time it gets written into the iclog.
  *
- * This function builds a vector for the changes in each log item in the
- * transaction. It then works out the length of the buffer needed for each log
- * item, allocates them and formats the vector for the item into the buffer.
- * The buffer is then attached to the log item are then inserted into the
- * Committed Item List for tracking until the next checkpoint is written out.
+ * This function takes the prepared log vectors attached to each log item, and
+ * formats the changes into the log vector buffer. The buffer it uses is
+ * dependent on the current state of the vector in the CIL - the shadow lv is
+ * guaranteed to be large enough for the current modification, but we will only
+ * use that if we can't reuse the existing lv. If we can't reuse the existing
+ * lv, then simple swap it out for the shadow lv. We don't free it - that is
+ * done lazily either by th enext modification or the freeing of the log item.
  *
  * We don't set up region headers during this process; we simply copy the
  * regions into the flat buffer. We can do this because we still have to do a
@@ -170,59 +326,29 @@ xlog_cil_insert_format_items(
 	list_for_each_entry(lidp, &tp->t_items, lid_trans) {
 		struct xfs_log_item *lip = lidp->lid_item;
 		struct xfs_log_vec *lv;
-		struct xfs_log_vec *old_lv;
-		int	niovecs = 0;
-		int	nbytes = 0;
-		int	buf_size;
+		struct xfs_log_vec *old_lv = NULL;
+		struct xfs_log_vec *shadow;
 		bool	ordered = false;
 
 		/* Skip items which aren't dirty in this transaction. */
 		if (!(lidp->lid_flags & XFS_LID_DIRTY))
 			continue;
 
-		/* get number of vecs and size of data to be stored */
-		lip->li_ops->iop_size(lip, &niovecs, &nbytes);
-
-		/* Skip items that do not have any vectors for writing */
-		if (!niovecs)
-			continue;
-
 		/*
-		 * Ordered items need to be tracked but we do not wish to write
-		 * them. We need a logvec to track the object, but we do not
-		 * need an iovec or buffer to be allocated for copying data.
+		 * The formatting size information is already attached to
+		 * the shadow lv on the log item.
 		 */
-		if (niovecs == XFS_LOG_VEC_ORDERED) {
+		shadow = lip->li_lv_shadow;
+		if (shadow->lv_buf_len == XFS_LOG_VEC_ORDERED)
 			ordered = true;
-			niovecs = 0;
-			nbytes = 0;
-		}
 
-		/*
-		 * We 64-bit align the length of each iovec so that the start
-		 * of the next one is naturally aligned.  We'll need to
-		 * account for that slack space here. Then round nbytes up
-		 * to 64-bit alignment so that the initial buffer alignment is
-		 * easy to calculate and verify.
-		 */
-		nbytes += niovecs * sizeof(uint64_t);
-		nbytes = round_up(nbytes, sizeof(uint64_t));
-
-		/* grab the old item if it exists for reservation accounting */
-		old_lv = lip->li_lv;
-
-		/*
-		 * The data buffer needs to start 64-bit aligned, so round up
-		 * that space to ensure we can align it appropriately and not
-		 * overrun the buffer.
-		 */
-		buf_size = nbytes +
-			   round_up((sizeof(struct xfs_log_vec) +
-				     niovecs * sizeof(struct xfs_log_iovec)),
-				    sizeof(uint64_t));
+		/* Skip items that do not have any vectors for writing */
+		if (!shadow->lv_niovecs && !ordered)
+			continue;
 
 		/* compare to existing item size */
-		if (lip->li_lv && buf_size <= lip->li_lv->lv_size) {
+		old_lv = lip->li_lv;
+		if (lip->li_lv && shadow->lv_size <= lip->li_lv->lv_size) {
 			/* same or smaller, optimise common overwrite case */
 			lv = lip->li_lv;
 			lv->lv_next = NULL;
@@ -236,32 +362,29 @@ xlog_cil_insert_format_items(
 			 */
 			*diff_iovecs -= lv->lv_niovecs;
 			*diff_len -= lv->lv_bytes;
+
+			/* Ensure the lv is set up according to ->iop_size */
+			lv->lv_niovecs = shadow->lv_niovecs;
+
+			/* reset the lv buffer information for new formatting */
+			lv->lv_buf_len = 0;
+			lv->lv_bytes = 0;
+			lv->lv_buf = (char *)lv +
+					xlog_cil_iovec_space(lv->lv_niovecs);
 		} else {
-			/* allocate new data chunk */
-			lv = kmem_zalloc(buf_size, KM_SLEEP|KM_NOFS);
+			/* switch to shadow buffer! */
+			lv = shadow;
 			lv->lv_item = lip;
-			lv->lv_size = buf_size;
 			if (ordered) {
 				/* track as an ordered logvec */
 				ASSERT(lip->li_lv == NULL);
-				lv->lv_buf_len = XFS_LOG_VEC_ORDERED;
 				goto insert;
 			}
-			lv->lv_iovecp = (struct xfs_log_iovec *)&lv[1];
 		}
 
-		/* Ensure the lv is set up according to ->iop_size */
-		lv->lv_niovecs = niovecs;
-
-		/* The allocated data region lies beyond the iovec region */
-		lv->lv_buf_len = 0;
-		lv->lv_bytes = 0;
-		lv->lv_buf = (char *)lv + buf_size - nbytes;
 		ASSERT(IS_ALIGNED((unsigned long)lv->lv_buf, sizeof(uint64_t)));
-
 		lip->li_ops->iop_format(lip, lv);
 insert:
-		ASSERT(lv->lv_buf_len <= nbytes);
 		xfs_cil_prepare_item(log, lv, old_lv, diff_len, diff_iovecs);
 	}
 }
@@ -783,6 +906,13 @@ xfs_log_commit_cil(
 	struct xlog		*log = mp->m_log;
 	struct xfs_cil		*cil = log->l_cilp;
 
+	/*
+	 * Do all necessary memory allocation before we lock the CIL.
+	 * This ensures the allocation does not deadlock with a CIL
+	 * push in memory reclaim (e.g. from kswapd).
+	 */
+	xlog_cil_alloc_shadow_bufs(log, tp);
+
 	/* lock out background commit */
 	down_read(&cil->xc_ctx_lock);
 

fs/xfs/xfs_trans.h

@@ -52,6 +52,7 @@ typedef struct xfs_log_item {
 	/* delayed logging */
 	struct list_head		li_cil;		/* CIL pointers */
 	struct xfs_log_vec		*li_lv;		/* active log vector */
+	struct xfs_log_vec		*li_lv_shadow;	/* standby vector */
 	xfs_lsn_t			li_seq;		/* CIL commit seq */
 } xfs_log_item_t;