xen-blkback: use balloon pages for all mappings

Using balloon pages for all granted pages allows us to simplify the
logic in blkback, especially in the xen_blkbk_map function, since now
we can decide if we want to map a grant persistently or not after we
have actually mapped it. This could not be done before because
persistent grants used ballooned pages, whereas non-persistent grants
used pages from the kernel.

This patch also introduces several changes, the first one is that the
list of free pages is no longer global, now each blkback instance has
it's own list of free pages that can be used to map grants. Also, a
run time parameter (max_buffer_pages) has been added in order to tune
the maximum number of free pages each blkback instance will keep in
it's buffer.
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Cc: xen-devel@lists.xen.org
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

Documentation/ABI/stable/sysfs-bus-xen-backend

@@ -73,3 +73,11 @@ KernelVersion:	3.0
 Contact:	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
 Description:
                 Number of sectors written by the frontend.
+
+What:           /sys/module/xen_blkback/parameters/max_buffer_pages
+Date:           March 2013
+KernelVersion:  3.10
+Contact:        Roger Pau Monné <roger.pau@citrix.com>
+Description:
+                Maximum number of free pages to keep in each block
+                backend buffer.

drivers/block/xen-blkback/blkback.c

@@ -63,6 +63,21 @@ static int xen_blkif_reqs = 64;
 module_param_named(reqs, xen_blkif_reqs, int, 0);
 MODULE_PARM_DESC(reqs, "Number of blkback requests to allocate");
 
+/*
+ * Maximum number of unused free pages to keep in the internal buffer.
+ * Setting this to a value too low will reduce memory used in each backend,
+ * but can have a performance penalty.
+ *
+ * A sane value is xen_blkif_reqs * BLKIF_MAX_SEGMENTS_PER_REQUEST, but can
+ * be set to a lower value that might degrade performance on some intensive
+ * IO workloads.
+ */
+
+static int xen_blkif_max_buffer_pages = 704;
+module_param_named(max_buffer_pages, xen_blkif_max_buffer_pages, int, 0644);
+MODULE_PARM_DESC(max_buffer_pages,
+"Maximum number of free pages to keep in each block backend buffer");
+
 /* Run-time switchable: /sys/module/blkback/parameters/ */
 static unsigned int log_stats;
 module_param(log_stats, int, 0644);
@@ -82,10 +97,14 @@ struct pending_req {
 	int			status;
 	struct list_head	free_list;
 	DECLARE_BITMAP(unmap_seg, BLKIF_MAX_SEGMENTS_PER_REQUEST);
+	struct page		*pages[BLKIF_MAX_SEGMENTS_PER_REQUEST];
 };
 
 #define BLKBACK_INVALID_HANDLE (~0)
 
+/* Number of free pages to remove on each call to free_xenballooned_pages */
+#define NUM_BATCH_FREE_PAGES 10
+
 struct xen_blkbk {
 	struct pending_req	*pending_reqs;
 	/* List of all 'pending_req' available */
@@ -93,8 +112,6 @@ struct xen_blkbk {
 	/* And its spinlock. */
 	spinlock_t		pending_free_lock;
 	wait_queue_head_t	pending_free_wq;
-	/* The list of all pages that are available. */
-	struct page		**pending_pages;
 	/* And the grant handles that are available. */
 	grant_handle_t		*pending_grant_handles;
 };
@@ -143,14 +160,66 @@ static inline int vaddr_pagenr(struct pending_req *req, int seg)
 		BLKIF_MAX_SEGMENTS_PER_REQUEST + seg;
 }
 
-#define pending_page(req, seg) pending_pages[vaddr_pagenr(req, seg)]
+static inline int get_free_page(struct xen_blkif *blkif, struct page **page)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&blkif->free_pages_lock, flags);
+	if (list_empty(&blkif->free_pages)) {
+		BUG_ON(blkif->free_pages_num != 0);
+		spin_unlock_irqrestore(&blkif->free_pages_lock, flags);
+		return alloc_xenballooned_pages(1, page, false);
+	}
+	BUG_ON(blkif->free_pages_num == 0);
+	page[0] = list_first_entry(&blkif->free_pages, struct page, lru);
+	list_del(&page[0]->lru);
+	blkif->free_pages_num--;
+	spin_unlock_irqrestore(&blkif->free_pages_lock, flags);
 
-static inline unsigned long vaddr(struct pending_req *req, int seg)
+	return 0;
+}
+
+static inline void put_free_pages(struct xen_blkif *blkif, struct page **page,
+                                  int num)
 {
-	unsigned long pfn = page_to_pfn(blkbk->pending_page(req, seg));
-	return (unsigned long)pfn_to_kaddr(pfn);
+	unsigned long flags;
+	int i;
+
+	spin_lock_irqsave(&blkif->free_pages_lock, flags);
+	for (i = 0; i < num; i++)
+		list_add(&page[i]->lru, &blkif->free_pages);
+	blkif->free_pages_num += num;
+	spin_unlock_irqrestore(&blkif->free_pages_lock, flags);
+}
+
+static inline void shrink_free_pagepool(struct xen_blkif *blkif, int num)
+{
+	/* Remove requested pages in batches of NUM_BATCH_FREE_PAGES */
+	struct page *page[NUM_BATCH_FREE_PAGES];
+	unsigned int num_pages = 0;
+	unsigned long flags;
+
+	spin_lock_irqsave(&blkif->free_pages_lock, flags);
+	while (blkif->free_pages_num > num) {
+		BUG_ON(list_empty(&blkif->free_pages));
+		page[num_pages] = list_first_entry(&blkif->free_pages,
+		                                   struct page, lru);
+		list_del(&page[num_pages]->lru);
+		blkif->free_pages_num--;
+		if (++num_pages == NUM_BATCH_FREE_PAGES) {
+			spin_unlock_irqrestore(&blkif->free_pages_lock, flags);
+			free_xenballooned_pages(num_pages, page);
+			spin_lock_irqsave(&blkif->free_pages_lock, flags);
+			num_pages = 0;
+		}
+	}
+	spin_unlock_irqrestore(&blkif->free_pages_lock, flags);
+	if (num_pages != 0)
+		free_xenballooned_pages(num_pages, page);
 }
 
+#define vaddr(page) ((unsigned long)pfn_to_kaddr(page_to_pfn(page)))
+
 #define pending_handle(_req, _seg) \
 	(blkbk->pending_grant_handles[vaddr_pagenr(_req, _seg)])
 
@@ -170,7 +239,7 @@ static void make_response(struct xen_blkif *blkif, u64 id,
 	     (n) = (&(pos)->node != NULL) ? rb_next(&(pos)->node) : NULL)
 
 
-static void add_persistent_gnt(struct rb_root *root,
+static int add_persistent_gnt(struct rb_root *root,
 			       struct persistent_gnt *persistent_gnt)
 {
 	struct rb_node **new = &(root->rb_node), *parent = NULL;
@@ -186,14 +255,15 @@ static void add_persistent_gnt(struct rb_root *root,
 		else if (persistent_gnt->gnt > this->gnt)
 			new = &((*new)->rb_right);
 		else {
-			pr_alert(DRV_PFX " trying to add a gref that's already in the tree\n");
-			BUG();
+			pr_alert_ratelimited(DRV_PFX " trying to add a gref that's already in the tree\n");
+			return -EINVAL;
 		}
 	}
 
 	/* Add new node and rebalance tree. */
 	rb_link_node(&(persistent_gnt->node), parent, new);
 	rb_insert_color(&(persistent_gnt->node), root);
+	return 0;
 }
 
 static struct persistent_gnt *get_persistent_gnt(struct rb_root *root,
@@ -215,7 +285,8 @@ static struct persistent_gnt *get_persistent_gnt(struct rb_root *root,
 	return NULL;
 }
 
-static void free_persistent_gnts(struct rb_root *root, unsigned int num)
+static void free_persistent_gnts(struct xen_blkif *blkif, struct rb_root *root,
+                                 unsigned int num)
 {
 	struct gnttab_unmap_grant_ref unmap[BLKIF_MAX_SEGMENTS_PER_REQUEST];
 	struct page *pages[BLKIF_MAX_SEGMENTS_PER_REQUEST];
@@ -240,7 +311,7 @@ static void free_persistent_gnts(struct rb_root *root, unsigned int num)
 			ret = gnttab_unmap_refs(unmap, NULL, pages,
 				segs_to_unmap);
 			BUG_ON(ret);
-			free_xenballooned_pages(segs_to_unmap, pages);
+			put_free_pages(blkif, pages, segs_to_unmap);
 			segs_to_unmap = 0;
 		}
 
@@ -422,13 +493,19 @@ int xen_blkif_schedule(void *arg)
 		if (do_block_io_op(blkif))
 			blkif->waiting_reqs = 1;
 
+		/* Shrink if we have more than xen_blkif_max_buffer_pages */
+		shrink_free_pagepool(blkif, xen_blkif_max_buffer_pages);
+
 		if (log_stats && time_after(jiffies, blkif->st_print))
 			print_stats(blkif);
 	}
 
+	/* Since we are shutting down remove all pages from the buffer */
+	shrink_free_pagepool(blkif, 0 /* All */);
+
 	/* Free all persistent grant pages */
 	if (!RB_EMPTY_ROOT(&blkif->persistent_gnts))
-		free_persistent_gnts(&blkif->persistent_gnts,
+		free_persistent_gnts(blkif, &blkif->persistent_gnts,
 			blkif->persistent_gnt_c);
 
 	BUG_ON(!RB_EMPTY_ROOT(&blkif->persistent_gnts));
@@ -457,23 +534,25 @@ static void xen_blkbk_unmap(struct pending_req *req)
 	struct page *pages[BLKIF_MAX_SEGMENTS_PER_REQUEST];
 	unsigned int i, invcount = 0;
 	grant_handle_t handle;
+	struct xen_blkif *blkif = req->blkif;
 	int ret;
 
 	for (i = 0; i < req->nr_pages; i++) {
 		if (!test_bit(i, req->unmap_seg))
 			continue;
 		handle = pending_handle(req, i);
+		pages[invcount] = req->pages[i];
 		if (handle == BLKBACK_INVALID_HANDLE)
 			continue;
-		gnttab_set_unmap_op(&unmap[invcount], vaddr(req, i),
+		gnttab_set_unmap_op(&unmap[invcount], vaddr(pages[invcount]),
 				    GNTMAP_host_map, handle);
 		pending_handle(req, i) = BLKBACK_INVALID_HANDLE;
-		pages[invcount] = virt_to_page(vaddr(req, i));
 		invcount++;
 	}
 
 	ret = gnttab_unmap_refs(unmap, NULL, pages, invcount);
 	BUG_ON(ret);
+	put_free_pages(blkif, pages, invcount);
 }
 
 static int xen_blkbk_map(struct blkif_request *req,
@@ -487,8 +566,7 @@ static int xen_blkbk_map(struct blkif_request *req,
 	struct persistent_gnt *persistent_gnt = NULL;
 	struct xen_blkif *blkif = pending_req->blkif;
 	phys_addr_t addr = 0;
-	int i, j;
-	bool new_map;
+	int i, seg_idx, new_map_idx;
 	int nseg = req->u.rw.nr_segments;
 	int segs_to_map = 0;
 	int ret = 0;
@@ -517,68 +595,16 @@ static int xen_blkbk_map(struct blkif_request *req,
 			 * We are using persistent grants and
 			 * the grant is already mapped
 			 */
-			new_map = false;
-		} else if (use_persistent_gnts &&
-			   blkif->persistent_gnt_c <
-			   max_mapped_grant_pages(blkif->blk_protocol)) {
-			/*
-			 * We are using persistent grants, the grant is
-			 * not mapped but we have room for it
-			 */
-			new_map = true;
-			persistent_gnt = kmalloc(
-				sizeof(struct persistent_gnt),
-				GFP_KERNEL);
-			if (!persistent_gnt)
-				return -ENOMEM;
-			if (alloc_xenballooned_pages(1, &persistent_gnt->page,
-			    false)) {
-				kfree(persistent_gnt);
-				return -ENOMEM;
-			}
-			persistent_gnt->gnt = req->u.rw.seg[i].gref;
-			persistent_gnt->handle = BLKBACK_INVALID_HANDLE;
-
-			pages_to_gnt[segs_to_map] =
-				persistent_gnt->page;
-			addr = (unsigned long) pfn_to_kaddr(
-				page_to_pfn(persistent_gnt->page));
-
-			add_persistent_gnt(&blkif->persistent_gnts,
-				persistent_gnt);
-			blkif->persistent_gnt_c++;
-			pr_debug(DRV_PFX " grant %u added to the tree of persistent grants, using %u/%u\n",
-				 persistent_gnt->gnt, blkif->persistent_gnt_c,
-				 max_mapped_grant_pages(blkif->blk_protocol));
-		} else {
-			/*
-			 * We are either using persistent grants and
-			 * hit the maximum limit of grants mapped,
-			 * or we are not using persistent grants.
-			 */
-			if (use_persistent_gnts &&
-				!blkif->vbd.overflow_max_grants) {
-				blkif->vbd.overflow_max_grants = 1;
-				pr_alert(DRV_PFX " domain %u, device %#x is using maximum number of persistent grants\n",
-					 blkif->domid, blkif->vbd.handle);
-			}
-			new_map = true;
-			pages[i] = blkbk->pending_page(pending_req, i);
-			addr = vaddr(pending_req, i);
-			pages_to_gnt[segs_to_map] =
-				blkbk->pending_page(pending_req, i);
-		}
-
-		if (persistent_gnt) {
 			pages[i] = persistent_gnt->page;
 			persistent_gnts[i] = persistent_gnt;
 		} else {
+			if (get_free_page(blkif, &pages[i]))
+				goto out_of_memory;
+			addr = vaddr(pages[i]);
+			pages_to_gnt[segs_to_map] = pages[i];
 			persistent_gnts[i] = NULL;
-		}
-
-		if (new_map) {
 			flags = GNTMAP_host_map;
-			if (!persistent_gnt &&
+			if (!use_persistent_gnts &&
 			    (pending_req->operation != BLKIF_OP_READ))
 				flags |= GNTMAP_readonly;
 			gnttab_set_map_op(&map[segs_to_map++], addr,
@@ -598,48 +624,81 @@ static int xen_blkbk_map(struct blkif_request *req,
 	 * the page from the other domain.
 	 */
 	bitmap_zero(pending_req->unmap_seg, BLKIF_MAX_SEGMENTS_PER_REQUEST);
-	for (i = 0, j = 0; i < nseg; i++) {
-		if (!persistent_gnts[i] ||
-		    persistent_gnts[i]->handle == BLKBACK_INVALID_HANDLE) {
+	for (seg_idx = 0, new_map_idx = 0; seg_idx < nseg; seg_idx++) {
+		if (!persistent_gnts[seg_idx]) {
 			/* This is a newly mapped grant */
-			BUG_ON(j >= segs_to_map);
-			if (unlikely(map[j].status != 0)) {
+			BUG_ON(new_map_idx >= segs_to_map);
+			if (unlikely(map[new_map_idx].status != 0)) {
 				pr_debug(DRV_PFX "invalid buffer -- could not remap it\n");
-				map[j].handle = BLKBACK_INVALID_HANDLE;
+				pending_handle(pending_req, seg_idx) = BLKBACK_INVALID_HANDLE;
 				ret |= 1;
-				if (persistent_gnts[i]) {
-					rb_erase(&persistent_gnts[i]->node,
-						 &blkif->persistent_gnts);
-					blkif->persistent_gnt_c--;
-					kfree(persistent_gnts[i]);
-					persistent_gnts[i] = NULL;
-				}
+				new_map_idx++;
+				/*
+				 * No need to set unmap_seg bit, since
+				 * we can not unmap this grant because
+				 * the handle is invalid.
+				 */
+				continue;
 			}
+			pending_handle(pending_req, seg_idx) = map[new_map_idx].handle;
+		} else {
+			/* This grant is persistent and already mapped */
+			goto next;
 		}
-		if (persistent_gnts[i]) {
-			if (persistent_gnts[i]->handle ==
-			    BLKBACK_INVALID_HANDLE) {
+		if (use_persistent_gnts &&
+		    blkif->persistent_gnt_c <
+		    max_mapped_grant_pages(blkif->blk_protocol)) {
+			/*
+			 * We are using persistent grants, the grant is
+			 * not mapped but we have room for it
+			 */
+			persistent_gnt = kmalloc(sizeof(struct persistent_gnt),
+				                 GFP_KERNEL);
+			if (!persistent_gnt) {
 				/*
-				 * If this is a new persistent grant
-				 * save the handler
+				 * If we don't have enough memory to
+				 * allocate the persistent_gnt struct
+				 * map this grant non-persistenly
 				 */
-				persistent_gnts[i]->handle = map[j++].handle;
+				goto next_unmap;
 			}
-			pending_handle(pending_req, i) =
-				persistent_gnts[i]->handle;
-
-			if (ret)
-				continue;
-		} else {
-			pending_handle(pending_req, i) = map[j++].handle;
-			bitmap_set(pending_req->unmap_seg, i, 1);
-
-			if (ret)
-				continue;
+			persistent_gnt->gnt = map[new_map_idx].ref;
+			persistent_gnt->handle = map[new_map_idx].handle;
+			persistent_gnt->page = pages[seg_idx];
+			if (add_persistent_gnt(&blkif->persistent_gnts,
+			                       persistent_gnt)) {
+				kfree(persistent_gnt);
+				persistent_gnt = NULL;
+				goto next_unmap;
+			}
+			blkif->persistent_gnt_c++;
+			pr_debug(DRV_PFX " grant %u added to the tree of persistent grants, using %u/%u\n",
+				 persistent_gnt->gnt, blkif->persistent_gnt_c,
+				 max_mapped_grant_pages(blkif->blk_protocol));
+			new_map_idx++;
+			goto next;
+		}
+		if (use_persistent_gnts && !blkif->vbd.overflow_max_grants) {
+			blkif->vbd.overflow_max_grants = 1;
+			pr_debug(DRV_PFX " domain %u, device %#x is using maximum number of persistent grants\n",
+			         blkif->domid, blkif->vbd.handle);
 		}
-		seg[i].offset = (req->u.rw.seg[i].first_sect << 9);
+next_unmap:
+		/*
+		 * We could not map this grant persistently, so use it as
+		 * a non-persistent grant.
+		 */
+		bitmap_set(pending_req->unmap_seg, seg_idx, 1);
+		new_map_idx++;
+next:
+		seg[seg_idx].offset = (req->u.rw.seg[seg_idx].first_sect << 9);
 	}
 	return ret;
+
+out_of_memory:
+	pr_alert(DRV_PFX "%s: out of memory\n", __func__);
+	put_free_pages(blkif, pages_to_gnt, segs_to_map);
+	return -ENOMEM;
 }
 
 static int dispatch_discard_io(struct xen_blkif *blkif,
@@ -863,7 +922,7 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
 	int operation;
 	struct blk_plug plug;
 	bool drain = false;
-	struct page *pages[BLKIF_MAX_SEGMENTS_PER_REQUEST];
+	struct page **pages = pending_req->pages;
 
 	switch (req->operation) {
 	case BLKIF_OP_READ:
@@ -1090,22 +1149,14 @@ static int __init xen_blkif_init(void)
 					xen_blkif_reqs, GFP_KERNEL);
 	blkbk->pending_grant_handles = kmalloc(sizeof(blkbk->pending_grant_handles[0]) *
 					mmap_pages, GFP_KERNEL);
-	blkbk->pending_pages         = kzalloc(sizeof(blkbk->pending_pages[0]) *
-					mmap_pages, GFP_KERNEL);
 
-	if (!blkbk->pending_reqs || !blkbk->pending_grant_handles ||
-	    !blkbk->pending_pages) {
+	if (!blkbk->pending_reqs || !blkbk->pending_grant_handles) {
 		rc = -ENOMEM;
 		goto out_of_memory;
 	}
 
 	for (i = 0; i < mmap_pages; i++) {
 		blkbk->pending_grant_handles[i] = BLKBACK_INVALID_HANDLE;
-		blkbk->pending_pages[i] = alloc_page(GFP_KERNEL);
-		if (blkbk->pending_pages[i] == NULL) {
-			rc = -ENOMEM;
-			goto out_of_memory;
-		}
 	}
 	rc = xen_blkif_interface_init();
 	if (rc)
@@ -1130,13 +1181,6 @@ static int __init xen_blkif_init(void)
  failed_init:
 	kfree(blkbk->pending_reqs);
 	kfree(blkbk->pending_grant_handles);
-	if (blkbk->pending_pages) {
-		for (i = 0; i < mmap_pages; i++) {
-			if (blkbk->pending_pages[i])
-				__free_page(blkbk->pending_pages[i]);
-		}
-		kfree(blkbk->pending_pages);
-	}
 	kfree(blkbk);
 	blkbk = NULL;
 	return rc;

drivers/block/xen-blkback/common.h

@@ -220,6 +220,11 @@ struct xen_blkif {
 	struct rb_root		persistent_gnts;
 	unsigned int		persistent_gnt_c;
 
+	/* buffer of free pages to map grant refs */
+	spinlock_t		free_pages_lock;
+	int			free_pages_num;
+	struct list_head	free_pages;
+
 	/* statistics */
 	unsigned long		st_print;
 	unsigned long long			st_rd_req;

drivers/block/xen-blkback/xenbus.c

@@ -118,6 +118,9 @@ static struct xen_blkif *xen_blkif_alloc(domid_t domid)
 	blkif->st_print = jiffies;
 	init_waitqueue_head(&blkif->waiting_to_free);
 	blkif->persistent_gnts.rb_node = NULL;
+	spin_lock_init(&blkif->free_pages_lock);
+	INIT_LIST_HEAD(&blkif->free_pages);
+	blkif->free_pages_num = 0;
 
 	return blkif;
 }