From: Qu Wenruo <wqu@suse.com>
To: linux-btrfs@vger.kernel.org
Subject: [PATCH] btrfs: remove some duplicated checks in btrfs_previous_*_item()
Date: Tue, 14 Dec 2021 15:14:11 +0800 [thread overview]
Message-ID: <20211214071411.48324-1-wqu@suse.com> (raw)
In btrfs_previous_item() and btrfs_previous_extent_item() we check
btrfs_header_nritems() in a loop.
But in fact we don't need to do it in a loop at all.
Firstly, if a tree block is empty, the whole tree is empty and nodes[0]
is the tree root.
We don't need to do anything and can exit immediately.
Secondly, the only timing we could get a slots[0] >= nritems is when the
function get called. After the first slots[0]--, the slot should always
be <= nritems.
So this patch will move all the nritems related checks out of the loop
by:
- Check nritems of nodes[0] to do a quick exit
- Sanitize path->slots[0] before entering the loop
I doubt if there is any caller setting path->slots[0] beyond
nritems + 1 (setting to nritems is possible when item is not found).
Sanitize path->slots[0] to nritems won't hurt anyway.
- Unconditionally reduce path->slots[0]
Since we're sure all tree blocks should not be empty, and
btrfs_prev_leaf() will return with path->slots[0] == nritems, we
can safely reduce slots[0] unconditionally.
Just keep an extra ASSERT() to make sure no tree block is empty.
Signed-off-by: Qu Wenruo <wqu@suse.com>
---
fs/btrfs/ctree.c | 52 +++++++++++++++++++++++++++++++++---------------
1 file changed, 36 insertions(+), 16 deletions(-)
diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index 781537692a4a..555345aed84d 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -4704,23 +4704,39 @@ int btrfs_previous_item(struct btrfs_root *root,
{
struct btrfs_key found_key;
struct extent_buffer *leaf;
- u32 nritems;
+ const u32 nritems = btrfs_header_nritems(path->nodes[0]);
int ret;
+ /*
+ * Check nritems first, if the tree is empty we exit immediately.
+ * And if this leave is not empty, none of the tree blocks of this root
+ * should be empty.
+ */
+ if (nritems == 0)
+ return 1;
+
+ /*
+ * If we're several slots beyond nritems, we reset slot to nritems,
+ * and it will be handled properly inside the loop.
+ */
+ if (unlikely(path->slots[0] > nritems))
+ path->slots[0] = nritems;
+
while (1) {
if (path->slots[0] == 0) {
ret = btrfs_prev_leaf(root, path);
if (ret != 0)
return ret;
- } else {
- path->slots[0]--;
}
leaf = path->nodes[0];
- nritems = btrfs_header_nritems(leaf);
- if (nritems == 0)
- return 1;
- if (path->slots[0] == nritems)
- path->slots[0]--;
+ ASSERT(btrfs_header_nritems(leaf));
+ /*
+ * This is for both regular case and above btrfs_prev_leaf() case.
+ * As btrfs_prev_leaf() will return with path->slots[0] == nritems,
+ * and we're sure no tree block is empty, we can go safely
+ * reduce slots[0] here.
+ */
+ path->slots[0]--;
btrfs_item_key_to_cpu(leaf, &found_key, path->slots[0]);
if (found_key.objectid < min_objectid)
@@ -4745,23 +4761,27 @@ int btrfs_previous_extent_item(struct btrfs_root *root,
{
struct btrfs_key found_key;
struct extent_buffer *leaf;
- u32 nritems;
+ const u32 nritems = btrfs_header_nritems(path->nodes[0]);
int ret;
+ /*
+ * Refer to btrfs_previous_item() for the reason of all nritems related
+ * checks/modifications.
+ */
+ if (nritems == 0)
+ return 1;
+ if (unlikely(path->slots[0] > nritems))
+ path->slots[0] = nritems;
+
while (1) {
if (path->slots[0] == 0) {
ret = btrfs_prev_leaf(root, path);
if (ret != 0)
return ret;
- } else {
- path->slots[0]--;
}
leaf = path->nodes[0];
- nritems = btrfs_header_nritems(leaf);
- if (nritems == 0)
- return 1;
- if (path->slots[0] == nritems)
- path->slots[0]--;
+ ASSERT(btrfs_header_nritems(leaf));
+ path->slots[0]--;
btrfs_item_key_to_cpu(leaf, &found_key, path->slots[0]);
if (found_key.objectid < min_objectid)
--
2.34.1
next reply other threads:[~2021-12-14 7:14 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-12-14 7:14 Qu Wenruo [this message]
2021-12-14 10:27 ` [PATCH] btrfs: remove some duplicated checks in btrfs_previous_*_item() Filipe Manana
2021-12-14 10:50 ` Qu Wenruo
2021-12-14 14:37 ` Josef Bacik
2021-12-14 23:33 ` Qu Wenruo
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