fix l2arc compression buffers leak

Commit log from FreeBSD:

    We have observed that arc_release() can be called concurrently with a
    l2arc in-flight write.  Also, we have observed that arc_hdr_destroy()
    can be called from arc_write_done() for a zio with ZIO_FLAG_IO_REWRITE
    flag in similar circumstances.

    Previously the l2arc headers would be freed while leaking their
    associated compression buffers.  Now the buffers are placed on
    l2arc_free_on_write list for delayed freeing.  This is similar to
    what was already done to arc buffers that were supposed to be freed
    concurrently with in-flight writes of those buffers.

    In addition to fixing the discovered leaks this change also adds
    some protective code to assert that a compression buffer associated
    with a l2arc header is never leaked.

    A new kstat l2_cdata_free_on_write is added.  It keeps a count
    of delayed compression buffer frees which previously would have
    been leaks.

    Tested by:  Vitalij Satanivskij <satan@ukr.net> et al
    Requested by:       many
    MFC after:  2 weeks
    Sponsored by:       HybridCluster / ClusterHQ

References:
    https://illumos.org/issues/5222
    https://github.com/freebsd/freebsd/commit/b98f85d
    http://thread.gmane.org/gmane.os.freebsd.current/155757/focus=155781
    http://lists.open-zfs.org/pipermail/developer/2014-January/000455.html
    http://lists.open-zfs.org/pipermail/developer/2014-February/000523.html

Ported-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3029

module/zfs/arc.c

@@ -304,6 +304,7 @@ typedef struct arc_stats {
 	kstat_named_t arcstat_l2_evict_lock_retry;
 	kstat_named_t arcstat_l2_evict_reading;
 	kstat_named_t arcstat_l2_free_on_write;
+	kstat_named_t arcstat_l2_cdata_free_on_write;
 	kstat_named_t arcstat_l2_abort_lowmem;
 	kstat_named_t arcstat_l2_cksum_bad;
 	kstat_named_t arcstat_l2_io_error;
@@ -392,6 +393,7 @@ static arc_stats_t arc_stats = {
 	{ "l2_evict_lock_retry",	KSTAT_DATA_UINT64 },
 	{ "l2_evict_reading",		KSTAT_DATA_UINT64 },
 	{ "l2_free_on_write",		KSTAT_DATA_UINT64 },
+	{ "l2_cdata_free_on_write",	KSTAT_DATA_UINT64 },
 	{ "l2_abort_lowmem",		KSTAT_DATA_UINT64 },
 	{ "l2_cksum_bad",		KSTAT_DATA_UINT64 },
 	{ "l2_io_error",		KSTAT_DATA_UINT64 },
@@ -1466,6 +1468,21 @@ arc_buf_add_ref(arc_buf_t *buf, void* tag)
 	    data, metadata, hits);
 }
 
+static void
+arc_buf_free_on_write(void *data, size_t size,
+    void (*free_func)(void *, size_t))
+{
+	l2arc_data_free_t *df;
+
+	df = kmem_alloc(sizeof (l2arc_data_free_t), KM_SLEEP);
+	df->l2df_data = data;
+	df->l2df_size = size;
+	df->l2df_func = free_func;
+	mutex_enter(&l2arc_free_on_write_mtx);
+	list_insert_head(l2arc_free_on_write, df);
+	mutex_exit(&l2arc_free_on_write_mtx);
+}
+
 /*
  * Free the arc data buffer.  If it is an l2arc write in progress,
  * the buffer is placed on l2arc_free_on_write to be freed later.
@@ -1476,14 +1493,7 @@ arc_buf_data_free(arc_buf_t *buf, void (*free_func)(void *, size_t))
 	arc_buf_hdr_t *hdr = buf->b_hdr;
 
 	if (HDR_L2_WRITING(hdr)) {
-		l2arc_data_free_t *df;
+		arc_buf_free_on_write(buf->b_data, hdr->b_size, free_func);
-		df = kmem_alloc(sizeof (l2arc_data_free_t), KM_SLEEP);
-		df->l2df_data = buf->b_data;
-		df->l2df_size = hdr->b_size;
-		df->l2df_func = free_func;
-		mutex_enter(&l2arc_free_on_write_mtx);
-		list_insert_head(l2arc_free_on_write, df);
-		mutex_exit(&l2arc_free_on_write_mtx);
 		ARCSTAT_BUMP(arcstat_l2_free_on_write);
 	} else {
 		free_func(buf->b_data, hdr->b_size);
@@ -1494,6 +1504,23 @@ arc_buf_data_free(arc_buf_t *buf, void (*free_func)(void *, size_t))
  * Free up buf->b_data and if 'remove' is set, then pull the
  * arc_buf_t off of the the arc_buf_hdr_t's list and free it.
  */
+static void
+arc_buf_l2_cdata_free(arc_buf_hdr_t *hdr)
+{
+	l2arc_buf_hdr_t *l2hdr = hdr->b_l2hdr;
+
+	ASSERT(MUTEX_HELD(&l2arc_buflist_mtx));
+
+	if (l2hdr->b_tmp_cdata == NULL)
+		return;
+
+	ASSERT(HDR_L2_WRITING(hdr));
+	arc_buf_free_on_write(l2hdr->b_tmp_cdata, hdr->b_size,
+	    zio_data_buf_free);
+	ARCSTAT_BUMP(arcstat_l2_cdata_free_on_write);
+	l2hdr->b_tmp_cdata = NULL;
+}
+
 static void
 arc_buf_destroy(arc_buf_t *buf, boolean_t recycle, boolean_t remove)
 {
@@ -1589,6 +1616,7 @@ arc_hdr_destroy(arc_buf_hdr_t *hdr)
 
 		if (l2hdr != NULL) {
 			list_remove(l2hdr->b_dev->l2ad_buflist, hdr);
+			arc_buf_l2_cdata_free(hdr);
 			ARCSTAT_INCR(arcstat_l2_size, -hdr->b_size);
 			ARCSTAT_INCR(arcstat_l2_asize, -l2hdr->b_asize);
 			vdev_space_update(l2hdr->b_dev->l2ad_vdev,
@@ -3602,6 +3630,7 @@ arc_release(arc_buf_t *buf, void *tag)
 	l2hdr = hdr->b_l2hdr;
 	if (l2hdr) {
 		mutex_enter(&l2arc_buflist_mtx);
+		arc_buf_l2_cdata_free(hdr);
 		hdr->b_l2hdr = NULL;
 		list_remove(l2hdr->b_dev->l2ad_buflist, hdr);
 	}
@@ -4843,6 +4872,11 @@ top:
 				ARCSTAT_INCR(arcstat_l2_asize, -abl2->b_asize);
 				bytes_evicted += abl2->b_asize;
 				ab->b_l2hdr = NULL;
+				/*
+				 * We are destroying l2hdr, so ensure that
+				 * its compressed buffer, if any, is not leaked.
+				 */
+				ASSERT(abl2->b_tmp_cdata == NULL);
 				kmem_cache_free(l2arc_hdr_cache, abl2);
 				arc_space_return(L2HDR_SIZE, ARC_SPACE_L2HDRS);
 				ARCSTAT_INCR(arcstat_l2_size, -ab->b_size);
@@ -5077,6 +5111,14 @@ l2arc_write_buffers(spa_t *spa, l2arc_dev_t *dev, uint64_t target_sz,
 		buf_data = l2hdr->b_tmp_cdata;
 		buf_sz = l2hdr->b_asize;
 
+		/*
+		 * If the data has not been compressed, then clear b_tmp_cdata
+		 * to make sure that it points only to a temporary compression
+		 * buffer.
+		 */
+		if (!L2ARC_IS_VALID_COMPRESS(l2hdr->b_compress))
+			l2hdr->b_tmp_cdata = NULL;
+
 		/* Compression may have squashed the buffer to zero length. */
 		if (buf_sz != 0) {
 			uint64_t buf_p_sz;
@@ -5267,15 +5309,18 @@ l2arc_release_cdata_buf(arc_buf_hdr_t *ab)
 {
 	l2arc_buf_hdr_t *l2hdr = ab->b_l2hdr;
 
-	if (l2hdr->b_compress == ZIO_COMPRESS_LZ4) {
+	ASSERT(L2ARC_IS_VALID_COMPRESS(l2hdr->b_compress));
+	if (l2hdr->b_compress != ZIO_COMPRESS_EMPTY) {
 		/*
 		 * If the data was compressed, then we've allocated a
 		 * temporary buffer for it, so now we need to release it.
 		 */
 		ASSERT(l2hdr->b_tmp_cdata != NULL);
 		zio_data_buf_free(l2hdr->b_tmp_cdata, ab->b_size);
+		l2hdr->b_tmp_cdata = NULL;
+	} else {
+		ASSERT(l2hdr->b_tmp_cdata == NULL);
 	}
-	l2hdr->b_tmp_cdata = NULL;
 }
 
 /*