[PATCH v3 0/3] Btrfs: relocation: Refactor build_backref_tree() using btrfs_backref_iterator infrastructure

[PATCH v3 0/3] Btrfs: relocation: Refactor build_backref_tree() using btrfs_backref_iterator infrastructure

[Qu Wenruo]

This is part 1 of the incoming refactor patches for build_backref_tree()

[THE PLAN]
The overall plan of refactoring build_backref_tree() is:
- Refactor how we iterate through backref items
  This patchset, the smallest I guess.

- Make build_backref_tree() easier to read.
  In short, that function is doing breadth-first-search to build a map
  which starts from one tree block, to all root nodes referring it.

  It involves backref iteration part, and a lot of backref cache only
  works.
  At least I hope to make this function less bulky and more structured.

- Make build_backref_tree() independent from relocation
  The hardest I guess.

  Current it even accepts reloc_control as its first parameter.
  Don't have a clear plan yet, but I guess at least I should make
  build_backref_tree() to do some more coverage, other than taking
  certain relocation-dependent shortcut.

[THIS PATCHSET]
For the patchset itself, the main purpose is to change how we iterate
through all backref items of one tree block.

The old way:

  path->search_commit_root = 1;
  path->skip_locking = 1;
  ret = btrfs_search_slot(NULL, extent_root, path, &key, 0, 0);
  ptr = btrfs_item_offset_nr()
  end = btrfs_item_end_nr()
  /* Inline item loop */
  while (ptr < end) {
	/* Handle each inline item here */
  }
  while (1) {
  	ret = btrfs_next_item();
	btrfs_item_key_to_cpu()
	if (key.objectid != bytenr ||
	    !(key.type == XXX || key.type == YYY)) 
		break;
	/* Handle each keyed item here */
  }
  
The new way:

  iterator = btrfs_backref_iterator_alloc();
  for (ret = btrfs_backref_iterator_start(iterator, bytenr);
       ret == 0; ret = btrfs_backref_iterator_next(iterator)) {
	/*
	 * Handle both keyed and inlined item here.
	 *
	 * We can use iterator->key to determine if it's inlined or
	 * keyed.
	 * Even for inlined item, it can be easily converted to keyed
	 * item, just like we did in build_backref_tree().
	 */
  }

Currently, only build_backref_tree() can utilize this infrastructure.

Backref.c has more requirement, as it needs to handle iteration for both
data and metadata, both commit root and current root.
And more importantly, backref.c uses depth first search, thus not a
perfect match for btrfs_backref_iterator.

Extra naming suggestion is welcomed.
The current naming, btrfs_backref_iterator_* looks pretty long to me
already.
Shorter naming would be much better.

Changelog:
v2:
- Fix a completion bug in btrfs_backref_iterator_next()
  It should be btrfs_extent_inline_ref_type().

v3:
- Comment and commit message update
- Move helper definitions to where they get first used
- Use helpers to replace some internal open code

Qu Wenruo (3):
  btrfs: backref: Introduce the skeleton of btrfs_backref_iterator
  btrfs: backref: Implement btrfs_backref_iterator_next()
  btrfs: relocation: Use btrfs_backref_iterator infrastructure

 fs/btrfs/backref.c    | 105 +++++++++++++++++++++++
 fs/btrfs/backref.h    |  96 +++++++++++++++++++++
 fs/btrfs/relocation.c | 195 ++++++++++++++----------------------------
 3 files changed, 265 insertions(+), 131 deletions(-)

-- 
2.25.0

[PATCH v3 1/3] btrfs: backref: Introduce the skeleton of btrfs_backref_iterator

[Qu Wenruo]

Due to the complex nature of btrfs extent tree, when we want to iterate
all backrefs of one extent, it involves quite a lot of work, like
searching the EXTENT_ITEM/METADATA_ITEM, iteration through inline and keyed
backrefs.

Normally this would result pretty complex code, something like:
  btrfs_search_slot()
  /* Ensure we are at EXTENT_ITEM/METADATA_ITEM */
  while (1) {	/* Loop for extent tree items */
	while (ptr < end) { /* Loop for inlined items */
		/* REAL WORK HERE */
	}
  next:
  	ret = btrfs_next_item()
	/* Ensure we're still at keyed item for specified bytenr */
  }

The idea of btrfs_backref_iterator is to avoid such complex and hard to
read code structure, but something like the following:

  iterator = btrfs_backref_iterator_alloc();
  ret = btrfs_backref_iterator_start(iterator, bytenr);
  if (ret < 0)
	goto out;
  for (; ; ret = btrfs_backref_iterator_next(iterator)) {
	/* REAL WORK HERE */
  }
  out:
  btrfs_backref_iterator_free(iterator);

This patch is just the skeleton + btrfs_backref_iterator_start() code.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/backref.c | 58 +++++++++++++++++++++++++++++++++++++++++++
 fs/btrfs/backref.h | 62 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 120 insertions(+)

diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
index e5d85311d5d5..8bd5e067831c 100644
--- a/fs/btrfs/backref.c
+++ b/fs/btrfs/backref.c
@@ -2252,3 +2252,61 @@ void free_ipath(struct inode_fs_paths *ipath)
 	kvfree(ipath->fspath);
 	kfree(ipath);
 }
+
+int btrfs_backref_iterator_start(struct btrfs_backref_iterator *iterator,
+				 u64 bytenr)
+{
+	struct btrfs_fs_info *fs_info = iterator->fs_info;
+	struct btrfs_path *path = iterator->path;
+	struct btrfs_extent_item *ei;
+	struct btrfs_key key;
+	int ret;
+
+	key.objectid = bytenr;
+	key.type = BTRFS_METADATA_ITEM_KEY;
+	key.offset = (u64)-1;
+
+	ret = btrfs_search_slot(NULL, fs_info->extent_root, &key, path, 0, 0);
+	if (ret < 0)
+		return ret;
+	if (ret == 0) {
+		ret = -EUCLEAN;
+		goto release;
+	}
+	if (path->slots[0] == 0) {
+		WARN_ON(IS_ENABLED(CONFIG_BTRFS_DEBUG));
+		ret = -EUCLEAN;
+		goto release;
+	}
+	path->slots[0]--;
+
+	btrfs_item_key_to_cpu(path->nodes[0], &key, path->slots[0]);
+	if (!(key.type == BTRFS_EXTENT_ITEM_KEY ||
+	      key.type == BTRFS_METADATA_ITEM_KEY) || key.objectid != bytenr) {
+		ret = -ENOENT;
+		goto release;
+	}
+	memcpy(&iterator->cur_key, &key, sizeof(key));
+	iterator->end_ptr = btrfs_item_end_nr(path->nodes[0], path->slots[0]);
+	iterator->item_ptr = btrfs_item_ptr_offset(path->nodes[0],
+						   path->slots[0]);
+	ei = btrfs_item_ptr(path->nodes[0], path->slots[0],
+			    struct btrfs_extent_item);
+
+	/*
+	 * Only support iteration on tree backref yet.
+	 *
+	 * This is extra precaustion for non skinny-metadata, where
+	 * EXTENT_ITEM is also used for tree blocks, that we can only use
+	 * extent flags to determine if it's a tree block.
+	 */
+	if (btrfs_extent_flags(path->nodes[0], ei) & BTRFS_EXTENT_FLAG_DATA) {
+		ret = -ENOTTY;
+		goto release;
+	}
+	iterator->cur_ptr = iterator->item_ptr + sizeof(*ei);
+	return 0;
+release:
+	btrfs_backref_iterator_release(iterator);
+	return ret;
+}
diff --git a/fs/btrfs/backref.h b/fs/btrfs/backref.h
index 777f61dc081e..fa73f02f14f6 100644
--- a/fs/btrfs/backref.h
+++ b/fs/btrfs/backref.h
@@ -74,4 +74,66 @@ struct prelim_ref {
 	u64 wanted_disk_byte;
 };
 
+/*
+ * Helper structure to help iterate backrefs of one extent.
+ *
+ * Now it only supports iteration for tree block in commit root.
+ */
+struct btrfs_backref_iterator {
+	u64 bytenr;
+	struct btrfs_path *path;
+	struct btrfs_fs_info *fs_info;
+	struct btrfs_key cur_key;
+	unsigned long item_ptr;
+	unsigned long cur_ptr;
+	unsigned long end_ptr;
+};
+
+static inline struct btrfs_backref_iterator *
+btrfs_backref_iterator_alloc(struct btrfs_fs_info *fs_info, gfp_t gfp_flag)
+{
+	struct btrfs_backref_iterator *ret;
+
+	ret = kzalloc(sizeof(*ret), gfp_flag);
+	if (!ret)
+		return NULL;
+
+	ret->path = btrfs_alloc_path();
+	if (!ret) {
+		kfree(ret);
+		return NULL;
+	}
+
+	/* Current backref iterator only supports iteration in commit root */
+	ret->path->search_commit_root = 1;
+	ret->path->skip_locking = 1;
+	ret->path->reada = READA_FORWARD;
+	ret->fs_info = fs_info;
+
+	return ret;
+}
+
+static inline void btrfs_backref_iterator_free(
+		struct btrfs_backref_iterator *iterator)
+{
+	if (!iterator)
+		return;
+	btrfs_free_path(iterator->path);
+	kfree(iterator);
+}
+
+int btrfs_backref_iterator_start(struct btrfs_backref_iterator *iterator,
+				 u64 bytenr);
+
+static inline void
+btrfs_backref_iterator_release(struct btrfs_backref_iterator *iterator)
+{
+	iterator->bytenr = 0;
+	iterator->item_ptr = 0;
+	iterator->cur_ptr = 0;
+	iterator->end_ptr = 0;
+	btrfs_release_path(iterator->path);
+	memset(&iterator->cur_key, 0, sizeof(iterator->cur_key));
+}
+
 #endif
-- 
2.25.0

[PATCH v3 2/3] btrfs: backref: Implement btrfs_backref_iterator_next()

[Qu Wenruo]

This function will go next inline/keyed backref for
btrfs_backref_iterator infrastructure.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/backref.c | 47 ++++++++++++++++++++++++++++++++++++++++++++++
 fs/btrfs/backref.h | 34 +++++++++++++++++++++++++++++++++
 2 files changed, 81 insertions(+)

diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
index 8bd5e067831c..fb0abe344851 100644
--- a/fs/btrfs/backref.c
+++ b/fs/btrfs/backref.c
@@ -2310,3 +2310,50 @@ int btrfs_backref_iterator_start(struct btrfs_backref_iterator *iterator,
 	btrfs_backref_iterator_release(iterator);
 	return ret;
 }
+
+int btrfs_backref_iterator_next(struct btrfs_backref_iterator *iterator)
+{
+	struct extent_buffer *eb = btrfs_backref_get_eb(iterator);
+	struct btrfs_path *path = iterator->path;
+	struct btrfs_extent_inline_ref *iref;
+	int ret;
+	u32 size;
+
+	if (btrfs_backref_iterator_is_inline_ref(iterator)) {
+		/* We're still inside the inline refs */
+		if (btrfs_backref_has_tree_block_info(iterator)) {
+			/* First tree block info */
+			size = sizeof(struct btrfs_tree_block_info);
+		} else {
+			/* Use inline ref type to determine the size */
+			int type;
+
+			iref = (struct btrfs_extent_inline_ref *)
+				(iterator->cur_ptr);
+			type = btrfs_extent_inline_ref_type(eb, iref);
+
+			size = btrfs_extent_inline_ref_size(type);
+		}
+		iterator->cur_ptr += size;
+		if (iterator->cur_ptr < iterator->end_ptr)
+			return 0;
+
+		/* All inline items iterated, fall through */
+	}
+	/* We're at keyed items, there is no inline item, just go next item */
+	ret = btrfs_next_item(iterator->fs_info->extent_root, iterator->path);
+	if (ret > 0 || ret < 0)
+		return ret;
+
+	btrfs_item_key_to_cpu(path->nodes[0], &iterator->cur_key,
+			      path->slots[0]);
+	if (iterator->cur_key.objectid != iterator->bytenr ||
+	    (iterator->cur_key.type != BTRFS_TREE_BLOCK_REF_KEY &&
+	     iterator->cur_key.type != BTRFS_SHARED_BLOCK_REF_KEY))
+		return 1;
+	iterator->item_ptr = btrfs_item_ptr_offset(path->nodes[0],
+						   path->slots[0]);
+	iterator->cur_ptr = iterator->item_ptr;
+	iterator->end_ptr = btrfs_item_end_nr(path->nodes[0], path->slots[0]);
+	return 0;
+}
diff --git a/fs/btrfs/backref.h b/fs/btrfs/backref.h
index fa73f02f14f6..016999339be1 100644
--- a/fs/btrfs/backref.h
+++ b/fs/btrfs/backref.h
@@ -122,8 +122,42 @@ static inline void btrfs_backref_iterator_free(
 	kfree(iterator);
 }
 
+static inline struct extent_buffer *
+btrfs_backref_get_eb(struct btrfs_backref_iterator *iterator)
+{
+	if (!iterator)
+		return NULL;
+	return iterator->path->nodes[0];
+}
+
+/*
+ * For metadata with EXTENT_ITEM key (non-skinny) case, the first inline data
+ * is btrfs_tree_block_info, without a btrfs_extent_inline_ref header.
+ *
+ * This helper is here to determine if that's the case.
+ */
+static inline bool btrfs_backref_has_tree_block_info(
+		struct btrfs_backref_iterator *iterator)
+{
+	if (iterator->cur_key.type == BTRFS_EXTENT_ITEM_KEY &&
+	    iterator->cur_ptr - iterator->item_ptr ==
+	    sizeof(struct btrfs_extent_item))
+		return true;
+	return false;
+}
+
 int btrfs_backref_iterator_start(struct btrfs_backref_iterator *iterator,
 				 u64 bytenr);
+int btrfs_backref_iterator_next(struct btrfs_backref_iterator *iterator);
+
+static inline bool
+btrfs_backref_iterator_is_inline_ref(struct btrfs_backref_iterator *iterator)
+{
+	if (iterator->cur_key.type == BTRFS_EXTENT_ITEM_KEY ||
+	    iterator->cur_key.type == BTRFS_METADATA_ITEM_KEY)
+		return true;
+	return false;
+}
 
 static inline void
 btrfs_backref_iterator_release(struct btrfs_backref_iterator *iterator)
-- 
2.25.0

[PATCH v3 3/3] btrfs: relocation: Use btrfs_backref_iterator infrastructure

[Qu Wenruo]

In the core function of relocation, build_backref_tree, it needs to
iterate all backref items of one tree block.

We don't really want to spend our code and reviewers' time to going
through tons of supportive code just for the backref walk.

Use btrfs_backref_iterator infrastructure to do the loop.

The backref items look would be much more easier to read:

	ret = btrfs_backref_iterator_start(iterator, cur->bytenr);
	for (; ret == 0; ret = btrfs_backref_iterator_next(iterator)) {
		/* The really important work */
	}

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/relocation.c | 195 ++++++++++++++----------------------------
 1 file changed, 64 insertions(+), 131 deletions(-)

diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
index b1365a516a25..499318acbfbf 100644
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -22,6 +22,7 @@
 #include "print-tree.h"
 #include "delalloc-space.h"
 #include "block-group.h"
+#include "backref.h"
 
 /*
  * backref_node, mapping_node and tree_block start with this
@@ -604,48 +605,6 @@ static struct btrfs_root *read_fs_root(struct btrfs_fs_info *fs_info,
 	return btrfs_get_fs_root(fs_info, &key, false);
 }
 
-static noinline_for_stack
-int find_inline_backref(struct extent_buffer *leaf, int slot,
-			unsigned long *ptr, unsigned long *end)
-{
-	struct btrfs_key key;
-	struct btrfs_extent_item *ei;
-	struct btrfs_tree_block_info *bi;
-	u32 item_size;
-
-	btrfs_item_key_to_cpu(leaf, &key, slot);
-
-	item_size = btrfs_item_size_nr(leaf, slot);
-	if (item_size < sizeof(*ei)) {
-		btrfs_print_v0_err(leaf->fs_info);
-		btrfs_handle_fs_error(leaf->fs_info, -EINVAL, NULL);
-		return 1;
-	}
-	ei = btrfs_item_ptr(leaf, slot, struct btrfs_extent_item);
-	WARN_ON(!(btrfs_extent_flags(leaf, ei) &
-		  BTRFS_EXTENT_FLAG_TREE_BLOCK));
-
-	if (key.type == BTRFS_EXTENT_ITEM_KEY &&
-	    item_size <= sizeof(*ei) + sizeof(*bi)) {
-		WARN_ON(item_size < sizeof(*ei) + sizeof(*bi));
-		return 1;
-	}
-	if (key.type == BTRFS_METADATA_ITEM_KEY &&
-	    item_size <= sizeof(*ei)) {
-		WARN_ON(item_size < sizeof(*ei));
-		return 1;
-	}
-
-	if (key.type == BTRFS_EXTENT_ITEM_KEY) {
-		bi = (struct btrfs_tree_block_info *)(ei + 1);
-		*ptr = (unsigned long)(bi + 1);
-	} else {
-		*ptr = (unsigned long)(ei + 1);
-	}
-	*end = (unsigned long)ei + item_size;
-	return 0;
-}
-
 /*
  * build backref tree for a given tree block. root of the backref tree
  * corresponds the tree block, leaves of the backref tree correspond
@@ -665,10 +624,9 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 					struct btrfs_key *node_key,
 					int level, u64 bytenr)
 {
+	struct btrfs_backref_iterator *iterator;
 	struct backref_cache *cache = &rc->backref_cache;
-	struct btrfs_path *path1; /* For searching extent root */
-	struct btrfs_path *path2; /* For searching parent of TREE_BLOCK_REF */
-	struct extent_buffer *eb;
+	struct btrfs_path *path; /* For searching parent of TREE_BLOCK_REF */
 	struct btrfs_root *root;
 	struct backref_node *cur;
 	struct backref_node *upper;
@@ -677,9 +635,6 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 	struct backref_node *exist = NULL;
 	struct backref_edge *edge;
 	struct rb_node *rb_node;
-	struct btrfs_key key;
-	unsigned long end;
-	unsigned long ptr;
 	LIST_HEAD(list); /* Pending edge list, upper node needs to be checked */
 	LIST_HEAD(useless);
 	int cowonly;
@@ -687,14 +642,16 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 	int err = 0;
 	bool need_check = true;
 
-	path1 = btrfs_alloc_path();
-	path2 = btrfs_alloc_path();
-	if (!path1 || !path2) {
+	iterator = btrfs_backref_iterator_alloc(rc->extent_root->fs_info,
+						GFP_NOFS);
+	if (!iterator)
+		return ERR_PTR(-ENOMEM);
+	path = btrfs_alloc_path();
+	if (!path) {
 		err = -ENOMEM;
 		goto out;
 	}
-	path1->reada = READA_FORWARD;
-	path2->reada = READA_FORWARD;
+	path->reada = READA_FORWARD;
 
 	node = alloc_backref_node(cache);
 	if (!node) {
@@ -707,25 +664,28 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 	node->lowest = 1;
 	cur = node;
 again:
-	end = 0;
-	ptr = 0;
-	key.objectid = cur->bytenr;
-	key.type = BTRFS_METADATA_ITEM_KEY;
-	key.offset = (u64)-1;
-
-	path1->search_commit_root = 1;
-	path1->skip_locking = 1;
-	ret = btrfs_search_slot(NULL, rc->extent_root, &key, path1,
-				0, 0);
+	ret = btrfs_backref_iterator_start(iterator, cur->bytenr);
 	if (ret < 0) {
 		err = ret;
 		goto out;
 	}
-	ASSERT(ret);
-	ASSERT(path1->slots[0]);
-
-	path1->slots[0]--;
 
+	/*
+	 * We skip the first btrfs_tree_block_info, as we don't use the key
+	 * stored in it, but fetch it from the tree block.
+	 */
+	if (btrfs_backref_has_tree_block_info(iterator)) {
+		ret = btrfs_backref_iterator_next(iterator);
+		if (ret < 0) {
+			err = ret;
+			goto out;
+		}
+		/* No extra backref? This means the tree block is corrupted */
+		if (ret > 0) {
+			err = -EUCLEAN;
+			goto out;
+		}
+	}
 	WARN_ON(cur->checked);
 	if (!list_empty(&cur->upper)) {
 		/*
@@ -747,42 +707,21 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 		exist = NULL;
 	}
 
-	while (1) {
-		cond_resched();
-		eb = path1->nodes[0];
-
-		if (ptr >= end) {
-			if (path1->slots[0] >= btrfs_header_nritems(eb)) {
-				ret = btrfs_next_leaf(rc->extent_root, path1);
-				if (ret < 0) {
-					err = ret;
-					goto out;
-				}
-				if (ret > 0)
-					break;
-				eb = path1->nodes[0];
-			}
+	for (; ret == 0; ret = btrfs_backref_iterator_next(iterator)) {
+		struct extent_buffer *eb;
+		struct btrfs_key key;
+		int type;
 
-			btrfs_item_key_to_cpu(eb, &key, path1->slots[0]);
-			if (key.objectid != cur->bytenr) {
-				WARN_ON(exist);
-				break;
-			}
+		cond_resched();
+		eb = btrfs_backref_get_eb(iterator);
 
-			if (key.type == BTRFS_EXTENT_ITEM_KEY ||
-			    key.type == BTRFS_METADATA_ITEM_KEY) {
-				ret = find_inline_backref(eb, path1->slots[0],
-							  &ptr, &end);
-				if (ret)
-					goto next;
-			}
-		}
+		key.objectid = iterator->bytenr;
+		if (btrfs_backref_iterator_is_inline_ref(iterator)) {
+			struct btrfs_extent_inline_ref *iref;
 
-		if (ptr < end) {
 			/* update key for inline back ref */
-			struct btrfs_extent_inline_ref *iref;
-			int type;
-			iref = (struct btrfs_extent_inline_ref *)ptr;
+			iref = (struct btrfs_extent_inline_ref *)
+				iterator->cur_ptr;
 			type = btrfs_get_extent_inline_ref_type(eb, iref,
 							BTRFS_REF_TYPE_BLOCK);
 			if (type == BTRFS_REF_TYPE_INVALID) {
@@ -791,9 +730,9 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 			}
 			key.type = type;
 			key.offset = btrfs_extent_inline_ref_offset(eb, iref);
-
-			WARN_ON(key.type != BTRFS_TREE_BLOCK_REF_KEY &&
-				key.type != BTRFS_SHARED_BLOCK_REF_KEY);
+		} else {
+			key.type = iterator->cur_key.type;
+			key.offset = iterator->cur_key.offset;
 		}
 
 		/*
@@ -806,7 +745,7 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 		     (key.type == BTRFS_SHARED_BLOCK_REF_KEY &&
 		      exist->bytenr == key.offset))) {
 			exist = NULL;
-			goto next;
+			continue;
 		}
 
 		/* SHARED_BLOCK_REF means key.offset is the parent bytenr */
@@ -852,7 +791,7 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 			edge->node[LOWER] = cur;
 			edge->node[UPPER] = upper;
 
-			goto next;
+			continue;
 		} else if (unlikely(key.type == BTRFS_EXTENT_REF_V0_KEY)) {
 			err = -EINVAL;
 			btrfs_print_v0_err(rc->extent_root->fs_info);
@@ -860,7 +799,7 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 					      NULL);
 			goto out;
 		} else if (key.type != BTRFS_TREE_BLOCK_REF_KEY) {
-			goto next;
+			continue;
 		}
 
 		/*
@@ -891,20 +830,20 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 		level = cur->level + 1;
 
 		/* Search the tree to find parent blocks referring the block. */
-		path2->search_commit_root = 1;
-		path2->skip_locking = 1;
-		path2->lowest_level = level;
-		ret = btrfs_search_slot(NULL, root, node_key, path2, 0, 0);
-		path2->lowest_level = 0;
+		path->search_commit_root = 1;
+		path->skip_locking = 1;
+		path->lowest_level = level;
+		ret = btrfs_search_slot(NULL, root, node_key, path, 0, 0);
+		path->lowest_level = 0;
 		if (ret < 0) {
 			err = ret;
 			goto out;
 		}
-		if (ret > 0 && path2->slots[level] > 0)
-			path2->slots[level]--;
+		if (ret > 0 && path->slots[level] > 0)
+			path->slots[level]--;
 
-		eb = path2->nodes[level];
-		if (btrfs_node_blockptr(eb, path2->slots[level]) !=
+		eb = path->nodes[level];
+		if (btrfs_node_blockptr(eb, path->slots[level]) !=
 		    cur->bytenr) {
 			btrfs_err(root->fs_info,
 	"couldn't find block (%llu) (level %d) in tree (%llu) with key (%llu %u %llu)",
@@ -920,7 +859,7 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 
 		/* Add all nodes and edges in the path */
 		for (; level < BTRFS_MAX_LEVEL; level++) {
-			if (!path2->nodes[level]) {
+			if (!path->nodes[level]) {
 				ASSERT(btrfs_root_bytenr(&root->root_item) ==
 				       lower->bytenr);
 				if (should_ignore_root(root))
@@ -936,7 +875,7 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 				goto out;
 			}
 
-			eb = path2->nodes[level];
+			eb = path->nodes[level];
 			rb_node = tree_search(&cache->rb_root, eb->start);
 			if (!rb_node) {
 				upper = alloc_backref_node(cache);
@@ -993,20 +932,14 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 			lower = upper;
 			upper = NULL;
 		}
-		btrfs_release_path(path2);
-next:
-		if (ptr < end) {
-			ptr += btrfs_extent_inline_ref_size(key.type);
-			if (ptr >= end) {
-				WARN_ON(ptr > end);
-				ptr = 0;
-				end = 0;
-			}
-		}
-		if (ptr >= end)
-			path1->slots[0]++;
+		btrfs_release_path(path);
+	}
+	if (ret < 0) {
+		err = ret;
+		goto out;
 	}
-	btrfs_release_path(path1);
+	ret = 0;
+	btrfs_backref_iterator_release(iterator);
 
 	cur->checked = 1;
 	WARN_ON(exist);
@@ -1124,8 +1057,8 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 		}
 	}
 out:
-	btrfs_free_path(path1);
-	btrfs_free_path(path2);
+	btrfs_backref_iterator_free(iterator);
+	btrfs_free_path(path);
 	if (err) {
 		while (!list_empty(&useless)) {
 			lower = list_entry(useless.next,
-- 
2.25.0

Re: [PATCH v3 1/3] btrfs: backref: Introduce the skeleton of btrfs_backref_iterator

[Nikolay Borisov]

On 17.02.20 г. 8:31 ч., Qu Wenruo wrote:
> Due to the complex nature of btrfs extent tree, when we want to iterate
> all backrefs of one extent, it involves quite a lot of work, like
> searching the EXTENT_ITEM/METADATA_ITEM, iteration through inline and keyed
> backrefs.
> 
> Normally this would result pretty complex code, something like:
>   btrfs_search_slot()
>   /* Ensure we are at EXTENT_ITEM/METADATA_ITEM */
>   while (1) {	/* Loop for extent tree items */
> 	while (ptr < end) { /* Loop for inlined items */
> 		/* REAL WORK HERE */
> 	}
>   next:
>   	ret = btrfs_next_item()
> 	/* Ensure we're still at keyed item for specified bytenr */
>   }
> 
> The idea of btrfs_backref_iterator is to avoid such complex and hard to
> read code structure, but something like the following:
> 
>   iterator = btrfs_backref_iterator_alloc();
>   ret = btrfs_backref_iterator_start(iterator, bytenr);
>   if (ret < 0)
> 	goto out;
>   for (; ; ret = btrfs_backref_iterator_next(iterator)) {
> 	/* REAL WORK HERE */
>   }
>   out:
>   btrfs_backref_iterator_free(iterator);
> 
> This patch is just the skeleton + btrfs_backref_iterator_start() code.
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>

Reviewed-by: Nikolay Borisov <nborisov@suse.com>

Re: [PATCH v3 2/3] btrfs: backref: Implement btrfs_backref_iterator_next()

[Nikolay Borisov]

On 17.02.20 г. 8:31 ч., Qu Wenruo wrote:
> This function will go next inline/keyed backref for
> btrfs_backref_iterator infrastructure.
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> ---
>  fs/btrfs/backref.c | 47 ++++++++++++++++++++++++++++++++++++++++++++++
>  fs/btrfs/backref.h | 34 +++++++++++++++++++++++++++++++++
>  2 files changed, 81 insertions(+)
> 
> diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
> index 8bd5e067831c..fb0abe344851 100644
> --- a/fs/btrfs/backref.c
> +++ b/fs/btrfs/backref.c
> @@ -2310,3 +2310,50 @@ int btrfs_backref_iterator_start(struct btrfs_backref_iterator *iterator,
>  	btrfs_backref_iterator_release(iterator);
>  	return ret;
>  }
> +
> +int btrfs_backref_iterator_next(struct btrfs_backref_iterator *iterator)

Document the return values: 0 in case there are more backerfs for the
given bytenr or 1 in case there are'nt. And a negative value in case of
error.

> +{
> +	struct extent_buffer *eb = btrfs_backref_get_eb(iterator);
> +	struct btrfs_path *path = iterator->path;
> +	struct btrfs_extent_inline_ref *iref;
> +	int ret;
> +	u32 size;
> +
> +	if (btrfs_backref_iterator_is_inline_ref(iterator)) {
> +		/* We're still inside the inline refs */
> +		if (btrfs_backref_has_tree_block_info(iterator)) {
> +			/* First tree block info */
> +			size = sizeof(struct btrfs_tree_block_info);
> +		} else {
> +			/* Use inline ref type to determine the size */
> +			int type;
> +
> +			iref = (struct btrfs_extent_inline_ref *)
> +				(iterator->cur_ptr);
> +			type = btrfs_extent_inline_ref_type(eb, iref);
> +
> +			size = btrfs_extent_inline_ref_size(type);
> +		}
> +		iterator->cur_ptr += size;
> +		if (iterator->cur_ptr < iterator->end_ptr)
> +			return 0;
> +
> +		/* All inline items iterated, fall through */
> +	}

This if could be rewritten as:
if (btrfs_backref_iterator_is_inline_ref(iterator) && iterator->cur_ptr
< iterator->end_ptr)

what this achieves is:

1. Clarity that this whole branch is executed only if we are within the
inline refs limits
2. It also optimises that function since in the current version, after
the last inline backref has been processed iterator->cur_ptr ==
iterator->end_ptr. On the next call to btrfs_backref_iterator_next you
will execute (needlessly)

(struct btrfs_extent_inline_ref *) (iterator->cur_ptr);
type = btrfs_extent_inline_ref_type(eb, iref);
size = btrfs_extent_inline_ref_size(type);
iterator->cur_ptr += size;
only to fail "if (iterator->cur_ptr < iterator->end_ptr)" check and
continue processing keyed items.

As a matter of fact you will be reading past the metadata_item  since
cur_ptr will be at the end of them and any deferences will read from the
next item this might not cause a crash but it's still wrong.


> +	/* We're at keyed items, there is no inline item, just go next item */
> +	ret = btrfs_next_item(iterator->fs_info->extent_root, iterator->path);
> +	if (ret > 0 || ret < 0)
> +		return ret;

nit: if (ret != 0) return ret;

<snip>

Re: [PATCH v3 2/3] btrfs: backref: Implement btrfs_backref_iterator_next()

[Qu Wenruo]

On 2020/2/17 下午6:47, Nikolay Borisov wrote:
>
>
> On 17.02.20 г. 8:31 ч., Qu Wenruo wrote:
>> This function will go next inline/keyed backref for
>> btrfs_backref_iterator infrastructure.
>>
>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>> ---
>>  fs/btrfs/backref.c | 47 ++++++++++++++++++++++++++++++++++++++++++++++
>>  fs/btrfs/backref.h | 34 +++++++++++++++++++++++++++++++++
>>  2 files changed, 81 insertions(+)
>>
>> diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
>> index 8bd5e067831c..fb0abe344851 100644
>> --- a/fs/btrfs/backref.c
>> +++ b/fs/btrfs/backref.c
>> @@ -2310,3 +2310,50 @@ int btrfs_backref_iterator_start(struct btrfs_backref_iterator *iterator,
>>  	btrfs_backref_iterator_release(iterator);
>>  	return ret;
>>  }
>> +
>> +int btrfs_backref_iterator_next(struct btrfs_backref_iterator *iterator)
>
> Document the return values: 0 in case there are more backerfs for the
> given bytenr or 1 in case there are'nt. And a negative value in case of
> error.
>
>> +{
>> +	struct extent_buffer *eb = btrfs_backref_get_eb(iterator);
>> +	struct btrfs_path *path = iterator->path;
>> +	struct btrfs_extent_inline_ref *iref;
>> +	int ret;
>> +	u32 size;
>> +
>> +	if (btrfs_backref_iterator_is_inline_ref(iterator)) {
>> +		/* We're still inside the inline refs */
>> +		if (btrfs_backref_has_tree_block_info(iterator)) {
>> +			/* First tree block info */
>> +			size = sizeof(struct btrfs_tree_block_info);
>> +		} else {
>> +			/* Use inline ref type to determine the size */
>> +			int type;
>> +
>> +			iref = (struct btrfs_extent_inline_ref *)
>> +				(iterator->cur_ptr);
>> +			type = btrfs_extent_inline_ref_type(eb, iref);
>> +
>> +			size = btrfs_extent_inline_ref_size(type);
>> +		}
>> +		iterator->cur_ptr += size;
>> +		if (iterator->cur_ptr < iterator->end_ptr)
>> +			return 0;
>> +
>> +		/* All inline items iterated, fall through */
>> +	}
>
> This if could be rewritten as:
> if (btrfs_backref_iterator_is_inline_ref(iterator) && iterator->cur_ptr
> < iterator->end_ptr)
>
> what this achieves is:
>
> 1. Clarity that this whole branch is executed only if we are within the
> inline refs limits
> 2. It also optimises that function since in the current version, after
> the last inline backref has been processed iterator->cur_ptr ==
> iterator->end_ptr. On the next call to btrfs_backref_iterator_next you
> will execute (needlessly)
>
> (struct btrfs_extent_inline_ref *) (iterator->cur_ptr);
> type = btrfs_extent_inline_ref_type(eb, iref);
> size = btrfs_extent_inline_ref_size(type);
> iterator->cur_ptr += size;
> only to fail "if (iterator->cur_ptr < iterator->end_ptr)" check and
> continue processing keyed items.
>
> As a matter of fact you will be reading past the metadata_item  since
> cur_ptr will be at the end of them and any deferences will read from the
> next item this might not cause a crash but it's still wrong.

This shouldn't happen, as we must ensure the cur_ptr < item_end for callers.

For the _next() call, the check after increased cur_ptr check it's OK.

But it's a problem for _start() call, as we may have a case where an
EXTENT_ITEM/METADATA_ITEM has no inlined ref.

I'll fix this in next version.

Thanks,
Qu

>
>
>> +	/* We're at keyed items, there is no inline item, just go next item */
>> +	ret = btrfs_next_item(iterator->fs_info->extent_root, iterator->path);
>> +	if (ret > 0 || ret < 0)
>> +		return ret;
>
> nit: if (ret != 0) return ret;
>
> <snip>
>

Re: [PATCH v3 3/3] btrfs: relocation: Use btrfs_backref_iterator infrastructure

[Nikolay Borisov]

On 17.02.20 г. 8:31 ч., Qu Wenruo wrote:
> In the core function of relocation, build_backref_tree, it needs to
> iterate all backref items of one tree block.
> 
> We don't really want to spend our code and reviewers' time to going
> through tons of supportive code just for the backref walk.
> 
> Use btrfs_backref_iterator infrastructure to do the loop.
> 
> The backref items look would be much more easier to read:
> 
> 	ret = btrfs_backref_iterator_start(iterator, cur->bytenr);
> 	for (; ret == 0; ret = btrfs_backref_iterator_next(iterator)) {
> 		/* The really important work */
> 	}
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>

Just some general observations:

1. You can name the iterator variable as iter, there is no loss of
meaning and less to type
2. The same applies to naming the function - you can shorten their name
by simply contracting iterator to iter.

Re: [PATCH v3 2/3] btrfs: backref: Implement btrfs_backref_iterator_next()

[Nikolay Borisov]

On 17.02.20 г. 13:29 ч., Qu Wenruo wrote:
> 
> 
> On 2020/2/17 下午6:47, Nikolay Borisov wrote:
>>
>>
>> On 17.02.20 г. 8:31 ч., Qu Wenruo wrote:
>>> This function will go next inline/keyed backref for
>>> btrfs_backref_iterator infrastructure.
>>>
>>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>>> ---
>>>  fs/btrfs/backref.c | 47 ++++++++++++++++++++++++++++++++++++++++++++++
>>>  fs/btrfs/backref.h | 34 +++++++++++++++++++++++++++++++++
>>>  2 files changed, 81 insertions(+)
>>>
>>> diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
>>> index 8bd5e067831c..fb0abe344851 100644
>>> --- a/fs/btrfs/backref.c
>>> +++ b/fs/btrfs/backref.c
>>> @@ -2310,3 +2310,50 @@ int btrfs_backref_iterator_start(struct btrfs_backref_iterator *iterator,
>>>  	btrfs_backref_iterator_release(iterator);
>>>  	return ret;
>>>  }
>>> +
>>> +int btrfs_backref_iterator_next(struct btrfs_backref_iterator *iterator)
>>
>> Document the return values: 0 in case there are more backerfs for the
>> given bytenr or 1 in case there are'nt. And a negative value in case of
>> error.
>>
>>> +{
>>> +	struct extent_buffer *eb = btrfs_backref_get_eb(iterator);
>>> +	struct btrfs_path *path = iterator->path;
>>> +	struct btrfs_extent_inline_ref *iref;
>>> +	int ret;
>>> +	u32 size;
>>> +
>>> +	if (btrfs_backref_iterator_is_inline_ref(iterator)) {
>>> +		/* We're still inside the inline refs */
>>> +		if (btrfs_backref_has_tree_block_info(iterator)) {
>>> +			/* First tree block info */
>>> +			size = sizeof(struct btrfs_tree_block_info);
>>> +		} else {
>>> +			/* Use inline ref type to determine the size */
>>> +			int type;
>>> +
>>> +			iref = (struct btrfs_extent_inline_ref *)
>>> +				(iterator->cur_ptr);
>>> +			type = btrfs_extent_inline_ref_type(eb, iref);
>>> +
>>> +			size = btrfs_extent_inline_ref_size(type);
>>> +		}
>>> +		iterator->cur_ptr += size;
>>> +		if (iterator->cur_ptr < iterator->end_ptr)
>>> +			return 0;
>>> +
>>> +		/* All inline items iterated, fall through */
>>> +	}
>>
>> This if could be rewritten as:
>> if (btrfs_backref_iterator_is_inline_ref(iterator) && iterator->cur_ptr
>> < iterator->end_ptr)
>>
>> what this achieves is:
>>
>> 1. Clarity that this whole branch is executed only if we are within the
>> inline refs limits
>> 2. It also optimises that function since in the current version, after
>> the last inline backref has been processed iterator->cur_ptr ==
>> iterator->end_ptr. On the next call to btrfs_backref_iterator_next you
>> will execute (needlessly)
>>
>> (struct btrfs_extent_inline_ref *) (iterator->cur_ptr);
>> type = btrfs_extent_inline_ref_type(eb, iref);
>> size = btrfs_extent_inline_ref_size(type);
>> iterator->cur_ptr += size;
>> only to fail "if (iterator->cur_ptr < iterator->end_ptr)" check and
>> continue processing keyed items.
>>
>> As a matter of fact you will be reading past the metadata_item  since
>> cur_ptr will be at the end of them and any deferences will read from the
>> next item this might not cause a crash but it's still wrong.
> 
> This shouldn't happen, as we must ensure the cur_ptr < item_end for callers.


How are you ensuring this? Before processing the last inline ref
cur_ptr  would be end_ptr - btrfs_extent_inline_ref_size(type);

After it's processed cur_ptr == end_ptr. THen you will do another call
to btrfs_backref_iterator_next which will do the same calculation? What
am I missing?

> 
> For the _next() call, the check after increased cur_ptr check it's OK.
> 
> But it's a problem for _start() call, as we may have a case where an
> EXTENT_ITEM/METADATA_ITEM has no inlined ref.
> 
> I'll fix this in next version.
> 
> Thanks,
> Qu
> 
>>
>>
>>> +	/* We're at keyed items, there is no inline item, just go next item */
>>> +	ret = btrfs_next_item(iterator->fs_info->extent_root, iterator->path);
>>> +	if (ret > 0 || ret < 0)
>>> +		return ret;
>>
>> nit: if (ret != 0) return ret;
>>
>> <snip>
>>

Re: [PATCH v3 2/3] btrfs: backref: Implement btrfs_backref_iterator_next()

[Qu Wenruo]

On 2020/2/17 下午7:42, Nikolay Borisov wrote:
>
>
> On 17.02.20 г. 13:29 ч., Qu Wenruo wrote:
>>
>>
>> On 2020/2/17 下午6:47, Nikolay Borisov wrote:
>>>
>>>
>>> On 17.02.20 г. 8:31 ч., Qu Wenruo wrote:
>>>> This function will go next inline/keyed backref for
>>>> btrfs_backref_iterator infrastructure.
>>>>
>>>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>>>> ---
>>>>  fs/btrfs/backref.c | 47 ++++++++++++++++++++++++++++++++++++++++++++++
>>>>  fs/btrfs/backref.h | 34 +++++++++++++++++++++++++++++++++
>>>>  2 files changed, 81 insertions(+)
>>>>
>>>> diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
>>>> index 8bd5e067831c..fb0abe344851 100644
>>>> --- a/fs/btrfs/backref.c
>>>> +++ b/fs/btrfs/backref.c
>>>> @@ -2310,3 +2310,50 @@ int btrfs_backref_iterator_start(struct btrfs_backref_iterator *iterator,
>>>>  	btrfs_backref_iterator_release(iterator);
>>>>  	return ret;
>>>>  }
>>>> +
>>>> +int btrfs_backref_iterator_next(struct btrfs_backref_iterator *iterator)
>>>
>>> Document the return values: 0 in case there are more backerfs for the
>>> given bytenr or 1 in case there are'nt. And a negative value in case of
>>> error.
>>>
>>>> +{
>>>> +	struct extent_buffer *eb = btrfs_backref_get_eb(iterator);
>>>> +	struct btrfs_path *path = iterator->path;
>>>> +	struct btrfs_extent_inline_ref *iref;
>>>> +	int ret;
>>>> +	u32 size;
>>>> +
>>>> +	if (btrfs_backref_iterator_is_inline_ref(iterator)) {
>>>> +		/* We're still inside the inline refs */
>>>> +		if (btrfs_backref_has_tree_block_info(iterator)) {
>>>> +			/* First tree block info */
>>>> +			size = sizeof(struct btrfs_tree_block_info);
>>>> +		} else {
>>>> +			/* Use inline ref type to determine the size */
>>>> +			int type;
>>>> +
>>>> +			iref = (struct btrfs_extent_inline_ref *)
>>>> +				(iterator->cur_ptr);
>>>> +			type = btrfs_extent_inline_ref_type(eb, iref);
>>>> +
>>>> +			size = btrfs_extent_inline_ref_size(type);
>>>> +		}
>>>> +		iterator->cur_ptr += size;
>>>> +		if (iterator->cur_ptr < iterator->end_ptr)
>>>> +			return 0;
>>>> +
>>>> +		/* All inline items iterated, fall through */
>>>> +	}
>>>
>>> This if could be rewritten as:
>>> if (btrfs_backref_iterator_is_inline_ref(iterator) && iterator->cur_ptr
>>> < iterator->end_ptr)
>>>
>>> what this achieves is:
>>>
>>> 1. Clarity that this whole branch is executed only if we are within the
>>> inline refs limits
>>> 2. It also optimises that function since in the current version, after
>>> the last inline backref has been processed iterator->cur_ptr ==
>>> iterator->end_ptr. On the next call to btrfs_backref_iterator_next you
>>> will execute (needlessly)
>>>
>>> (struct btrfs_extent_inline_ref *) (iterator->cur_ptr);
>>> type = btrfs_extent_inline_ref_type(eb, iref);
>>> size = btrfs_extent_inline_ref_size(type);
>>> iterator->cur_ptr += size;
>>> only to fail "if (iterator->cur_ptr < iterator->end_ptr)" check and
>>> continue processing keyed items.
>>>
>>> As a matter of fact you will be reading past the metadata_item  since
>>> cur_ptr will be at the end of them and any deferences will read from the
>>> next item this might not cause a crash but it's still wrong.
>>
>> This shouldn't happen, as we must ensure the cur_ptr < item_end for callers.
>
>
> How are you ensuring this? Before processing the last inline ref
> cur_ptr  would be end_ptr - btrfs_extent_inline_ref_size(type);

Firstly, in _start() call, we can easily check if we have any inline refs.

If no, search next item.
If yes, return cur_ptr which points to the current inline extent ref.

Secondly, in _next() call, we keep current check. Increase cur_ptr, then
check against ptr_end.

So that, all backref_iter callers will get a cur_ptr that is always
smaller than ptr_end.

Thanks,
Qu
>
> After it's processed cur_ptr == end_ptr. THen you will do another call
> to btrfs_backref_iterator_next which will do the same calculation? What
> am I missing?
>
>>
>> For the _next() call, the check after increased cur_ptr check it's OK.
>>
>> But it's a problem for _start() call, as we may have a case where an
>> EXTENT_ITEM/METADATA_ITEM has no inlined ref.
>>
>> I'll fix this in next version.
>>
>> Thanks,
>> Qu
>>
>>>
>>>
>>>> +	/* We're at keyed items, there is no inline item, just go next item */
>>>> +	ret = btrfs_next_item(iterator->fs_info->extent_root, iterator->path);
>>>> +	if (ret > 0 || ret < 0)
>>>> +		return ret;
>>>
>>> nit: if (ret != 0) return ret;
>>>
>>> <snip>
>>>

Re: [PATCH v3 2/3] btrfs: backref: Implement btrfs_backref_iterator_next()

[Nikolay Borisov]

On 17.02.20 г. 13:45 ч., Qu Wenruo wrote:
> 
> 
> On 2020/2/17 下午7:42, Nikolay Borisov wrote:
>>
>>
>> On 17.02.20 г. 13:29 ч., Qu Wenruo wrote:
>>>
>>>
>>> On 2020/2/17 下午6:47, Nikolay Borisov wrote:
>>>>
>>>>
>>>> On 17.02.20 г. 8:31 ч., Qu Wenruo wrote:
>>>>> This function will go next inline/keyed backref for
>>>>> btrfs_backref_iterator infrastructure.
>>>>>
>>>>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>>>>> ---
>>>>>  fs/btrfs/backref.c | 47 ++++++++++++++++++++++++++++++++++++++++++++++
>>>>>  fs/btrfs/backref.h | 34 +++++++++++++++++++++++++++++++++
>>>>>  2 files changed, 81 insertions(+)
>>>>>
>>>>> diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
>>>>> index 8bd5e067831c..fb0abe344851 100644
>>>>> --- a/fs/btrfs/backref.c
>>>>> +++ b/fs/btrfs/backref.c
>>>>> @@ -2310,3 +2310,50 @@ int btrfs_backref_iterator_start(struct btrfs_backref_iterator *iterator,
>>>>>  	btrfs_backref_iterator_release(iterator);
>>>>>  	return ret;
>>>>>  }
>>>>> +
>>>>> +int btrfs_backref_iterator_next(struct btrfs_backref_iterator *iterator)
>>>>
>>>> Document the return values: 0 in case there are more backerfs for the
>>>> given bytenr or 1 in case there are'nt. And a negative value in case of
>>>> error.
>>>>
>>>>> +{
>>>>> +	struct extent_buffer *eb = btrfs_backref_get_eb(iterator);
>>>>> +	struct btrfs_path *path = iterator->path;
>>>>> +	struct btrfs_extent_inline_ref *iref;
>>>>> +	int ret;
>>>>> +	u32 size;
>>>>> +
>>>>> +	if (btrfs_backref_iterator_is_inline_ref(iterator)) {
>>>>> +		/* We're still inside the inline refs */
>>>>> +		if (btrfs_backref_has_tree_block_info(iterator)) {
>>>>> +			/* First tree block info */
>>>>> +			size = sizeof(struct btrfs_tree_block_info);
>>>>> +		} else {
>>>>> +			/* Use inline ref type to determine the size */
>>>>> +			int type;
>>>>> +
>>>>> +			iref = (struct btrfs_extent_inline_ref *)
>>>>> +				(iterator->cur_ptr);
>>>>> +			type = btrfs_extent_inline_ref_type(eb, iref);
>>>>> +
>>>>> +			size = btrfs_extent_inline_ref_size(type);
>>>>> +		}
>>>>> +		iterator->cur_ptr += size;
>>>>> +		if (iterator->cur_ptr < iterator->end_ptr)
>>>>> +			return 0;
>>>>> +
>>>>> +		/* All inline items iterated, fall through */
>>>>> +	}
>>>>
>>>> This if could be rewritten as:
>>>> if (btrfs_backref_iterator_is_inline_ref(iterator) && iterator->cur_ptr
>>>> < iterator->end_ptr)
>>>>
>>>> what this achieves is:
>>>>
>>>> 1. Clarity that this whole branch is executed only if we are within the
>>>> inline refs limits
>>>> 2. It also optimises that function since in the current version, after
>>>> the last inline backref has been processed iterator->cur_ptr ==
>>>> iterator->end_ptr. On the next call to btrfs_backref_iterator_next you
>>>> will execute (needlessly)
>>>>
>>>> (struct btrfs_extent_inline_ref *) (iterator->cur_ptr);
>>>> type = btrfs_extent_inline_ref_type(eb, iref);
>>>> size = btrfs_extent_inline_ref_size(type);
>>>> iterator->cur_ptr += size;
>>>> only to fail "if (iterator->cur_ptr < iterator->end_ptr)" check and
>>>> continue processing keyed items.
>>>>
>>>> As a matter of fact you will be reading past the metadata_item  since
>>>> cur_ptr will be at the end of them and any deferences will read from the
>>>> next item this might not cause a crash but it's still wrong.
>>>
>>> This shouldn't happen, as we must ensure the cur_ptr < item_end for callers.
>>
>>
>> How are you ensuring this? Before processing the last inline ref
>> cur_ptr  would be end_ptr - btrfs_extent_inline_ref_size(type);
> 
> Firstly, in _start() call, we can easily check if we have any inline refs.
> 
> If no, search next item.
> If yes, return cur_ptr which points to the current inline extent ref.
> 
> Secondly, in _next() call, we keep current check. Increase cur_ptr, then
> check against ptr_end.
> 
> So that, all backref_iter callers will get a cur_ptr that is always
> smaller than ptr_end.

Apparently not, btrfs/003 with the following assert:

diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
index fb0abe344851..403a75f0c99c 100644
--- a/fs/btrfs/backref.c
+++ b/fs/btrfs/backref.c
@@ -2328,6 +2328,7 @@ int btrfs_backref_iterator_next(struct
btrfs_backref_iterator *iterator)
                        /* Use inline ref type to determine the size */
                        int type;

+                       ASSERT(iterator->cur_ptr < iterator->end_ptr);
                        iref = (struct btrfs_extent_inline_ref *)
                                (iterator->cur_ptr);
                        type = btrfs_extent_inline_ref_type(eb, iref);


Trigger:

[   58.884441] assertion failed: iterator->cur_ptr < iterator->end_ptr,
in fs/btrfs/backref.c:2331


> 
> Thanks,
> Qu
>>
>> After it's processed cur_ptr == end_ptr. THen you will do another call
>> to btrfs_backref_iterator_next which will do the same calculation? What
>> am I missing?
>>
>>>
>>> For the _next() call, the check after increased cur_ptr check it's OK.
>>>
>>> But it's a problem for _start() call, as we may have a case where an
>>> EXTENT_ITEM/METADATA_ITEM has no inlined ref.
>>>
>>> I'll fix this in next version.
>>>
>>> Thanks,
>>> Qu
>>>
>>>>
>>>>
>>>>> +	/* We're at keyed items, there is no inline item, just go next item */
>>>>> +	ret = btrfs_next_item(iterator->fs_info->extent_root, iterator->path);
>>>>> +	if (ret > 0 || ret < 0)
>>>>> +		return ret;
>>>>
>>>> nit: if (ret != 0) return ret;
>>>>
>>>> <snip>
>>>>

Re: [PATCH v3 3/3] btrfs: relocation: Use btrfs_backref_iterator infrastructure

[Nikolay Borisov]

On 17.02.20 г. 8:31 ч., Qu Wenruo wrote:
> In the core function of relocation, build_backref_tree, it needs to
> iterate all backref items of one tree block.
> 
> We don't really want to spend our code and reviewers' time to going
> through tons of supportive code just for the backref walk.
> 
> Use btrfs_backref_iterator infrastructure to do the loop.
> 
> The backref items look would be much more easier to read:
> 
> 	ret = btrfs_backref_iterator_start(iterator, cur->bytenr);
> 	for (; ret == 0; ret = btrfs_backref_iterator_next(iterator)) {
> 		/* The really important work */
> 	}
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>

Even with your branch this patch is triggering warnings on btrfs/187 :


[ 1053.603321] WARNING: CPU: 1 PID: 3110 at fs/btrfs/relocation.c:945
build_backref_tree+0x270c/0x50b0

That's         WARN_ON(exist);

Re: [PATCH v3 2/3] btrfs: backref: Implement btrfs_backref_iterator_next()

[Qu Wenruo]

On 2020/2/17 下午10:13, Nikolay Borisov wrote:
> 
> 
> On 17.02.20 г. 13:45 ч., Qu Wenruo wrote:
>>
>>
>> On 2020/2/17 下午7:42, Nikolay Borisov wrote:
>>>
>>>
>>> On 17.02.20 г. 13:29 ч., Qu Wenruo wrote:
>>>>
>>>>
>>>> On 2020/2/17 下午6:47, Nikolay Borisov wrote:
>>>>>
>>>>>
>>>>> On 17.02.20 г. 8:31 ч., Qu Wenruo wrote:
>>>>>> This function will go next inline/keyed backref for
>>>>>> btrfs_backref_iterator infrastructure.
>>>>>>
>>>>>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>>>>>> ---
>>>>>>  fs/btrfs/backref.c | 47 ++++++++++++++++++++++++++++++++++++++++++++++
>>>>>>  fs/btrfs/backref.h | 34 +++++++++++++++++++++++++++++++++
>>>>>>  2 files changed, 81 insertions(+)
>>>>>>
>>>>>> diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
>>>>>> index 8bd5e067831c..fb0abe344851 100644
>>>>>> --- a/fs/btrfs/backref.c
>>>>>> +++ b/fs/btrfs/backref.c
>>>>>> @@ -2310,3 +2310,50 @@ int btrfs_backref_iterator_start(struct btrfs_backref_iterator *iterator,
>>>>>>  	btrfs_backref_iterator_release(iterator);
>>>>>>  	return ret;
>>>>>>  }
>>>>>> +
>>>>>> +int btrfs_backref_iterator_next(struct btrfs_backref_iterator *iterator)
>>>>>
>>>>> Document the return values: 0 in case there are more backerfs for the
>>>>> given bytenr or 1 in case there are'nt. And a negative value in case of
>>>>> error.
>>>>>
>>>>>> +{
>>>>>> +	struct extent_buffer *eb = btrfs_backref_get_eb(iterator);
>>>>>> +	struct btrfs_path *path = iterator->path;
>>>>>> +	struct btrfs_extent_inline_ref *iref;
>>>>>> +	int ret;
>>>>>> +	u32 size;
>>>>>> +
>>>>>> +	if (btrfs_backref_iterator_is_inline_ref(iterator)) {
>>>>>> +		/* We're still inside the inline refs */
>>>>>> +		if (btrfs_backref_has_tree_block_info(iterator)) {
>>>>>> +			/* First tree block info */
>>>>>> +			size = sizeof(struct btrfs_tree_block_info);
>>>>>> +		} else {
>>>>>> +			/* Use inline ref type to determine the size */
>>>>>> +			int type;
>>>>>> +
>>>>>> +			iref = (struct btrfs_extent_inline_ref *)
>>>>>> +				(iterator->cur_ptr);
>>>>>> +			type = btrfs_extent_inline_ref_type(eb, iref);
>>>>>> +
>>>>>> +			size = btrfs_extent_inline_ref_size(type);
>>>>>> +		}
>>>>>> +		iterator->cur_ptr += size;
>>>>>> +		if (iterator->cur_ptr < iterator->end_ptr)
>>>>>> +			return 0;
>>>>>> +
>>>>>> +		/* All inline items iterated, fall through */
>>>>>> +	}
>>>>>
>>>>> This if could be rewritten as:
>>>>> if (btrfs_backref_iterator_is_inline_ref(iterator) && iterator->cur_ptr
>>>>> < iterator->end_ptr)
>>>>>
>>>>> what this achieves is:
>>>>>
>>>>> 1. Clarity that this whole branch is executed only if we are within the
>>>>> inline refs limits
>>>>> 2. It also optimises that function since in the current version, after
>>>>> the last inline backref has been processed iterator->cur_ptr ==
>>>>> iterator->end_ptr. On the next call to btrfs_backref_iterator_next you
>>>>> will execute (needlessly)
>>>>>
>>>>> (struct btrfs_extent_inline_ref *) (iterator->cur_ptr);
>>>>> type = btrfs_extent_inline_ref_type(eb, iref);
>>>>> size = btrfs_extent_inline_ref_size(type);
>>>>> iterator->cur_ptr += size;
>>>>> only to fail "if (iterator->cur_ptr < iterator->end_ptr)" check and
>>>>> continue processing keyed items.
>>>>>
>>>>> As a matter of fact you will be reading past the metadata_item  since
>>>>> cur_ptr will be at the end of them and any deferences will read from the
>>>>> next item this might not cause a crash but it's still wrong.
>>>>
>>>> This shouldn't happen, as we must ensure the cur_ptr < item_end for callers.
>>>
>>>
>>> How are you ensuring this? Before processing the last inline ref
>>> cur_ptr  would be end_ptr - btrfs_extent_inline_ref_size(type);
>>
>> Firstly, in _start() call, we can easily check if we have any inline refs.
>>
>> If no, search next item.
>> If yes, return cur_ptr which points to the current inline extent ref.
>>
>> Secondly, in _next() call, we keep current check. Increase cur_ptr, then
>> check against ptr_end.
>>
>> So that, all backref_iter callers will get a cur_ptr that is always
>> smaller than ptr_end.
> 
> Apparently not, btrfs/003 with the following assert:
> 
> diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
> index fb0abe344851..403a75f0c99c 100644
> --- a/fs/btrfs/backref.c
> +++ b/fs/btrfs/backref.c
> @@ -2328,6 +2328,7 @@ int btrfs_backref_iterator_next(struct
> btrfs_backref_iterator *iterator)
>                         /* Use inline ref type to determine the size */
>                         int type;
> 
> +                       ASSERT(iterator->cur_ptr < iterator->end_ptr);
>                         iref = (struct btrfs_extent_inline_ref *)
>                                 (iterator->cur_ptr);
>                         type = btrfs_extent_inline_ref_type(eb, iref);

Exactly what I said, in _start() there is not enough check to ensure
cur_ptr is always smaller than end_ptr.

Thus it triggers the ASSERT().
Will fix in next version.

Thanks,
Qu
> 
> 
> Trigger:
> 
> [   58.884441] assertion failed: iterator->cur_ptr < iterator->end_ptr,
> in fs/btrfs/backref.c:2331
> 
> 
>>
>> Thanks,
>> Qu
>>>
>>> After it's processed cur_ptr == end_ptr. THen you will do another call
>>> to btrfs_backref_iterator_next which will do the same calculation? What
>>> am I missing?
>>>
>>>>
>>>> For the _next() call, the check after increased cur_ptr check it's OK.
>>>>
>>>> But it's a problem for _start() call, as we may have a case where an
>>>> EXTENT_ITEM/METADATA_ITEM has no inlined ref.
>>>>
>>>> I'll fix this in next version.
>>>>
>>>> Thanks,
>>>> Qu
>>>>
>>>>>
>>>>>
>>>>>> +	/* We're at keyed items, there is no inline item, just go next item */
>>>>>> +	ret = btrfs_next_item(iterator->fs_info->extent_root, iterator->path);
>>>>>> +	if (ret > 0 || ret < 0)
>>>>>> +		return ret;
>>>>>
>>>>> nit: if (ret != 0) return ret;
>>>>>
>>>>> <snip>
>>>>>