[RFC,v3,5/5] libfs: Refactor offset_iterate_dir()

Commit Message

Chuck Lever Nov. 27, 2024, 3:28 p.m. UTC

From: Chuck Lever <[e-mail]>

This line in offset_iterate_dir():

		ctx->pos = dentry2offset(dentry) + 1;

assumes that the next child entry has an offset value that is
greater than the current child entry. Since directory offsets are
actually cookies, this heuristic is not always correct.

We have tested the current code with a limited offset range to see
if this is an operational problem. It doesn't seem to be, but doing
a "+ 1" on what is supposed to be an opaque cookie is very likely
wrong and brittle.

Instead of using the mtree to emit entries in the order of their
offset values, use it only to map the initial ctx->pos to a starting
entry. Then use the directory's d_children list, which is already
maintained by the dcache, to find the next child to emit, as the
simple cursor-based implementation still does.

Signed-off-by: Chuck Lever <[e-mail]>
---
 fs/libfs.c | 95 ++++++++++++++++++++++++++++++++++++++++++------------
 1 file changed, 74 insertions(+), 21 deletions(-)

Patch

diff --git a/fs/libfs.c b/fs/libfs.c
index 0deff5390abb..2616421bbe0e 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -241,9 +241,9 @@  const struct inode_operations simple_dir_inode_operations = {
 };
 EXPORT_SYMBOL(simple_dir_inode_operations);
 
-/* 0 is '.', 1 is '..', so always start with offset 2 or more */
 enum {
-	DIR_OFFSET_MIN	= 2,
+	DIR_OFFSET_FIRST	= 2,	/* seek to the first real entry */
+	DIR_OFFSET_MIN		= 3,	/* minimum allocated offset value */
 };
 
 static void offset_set(struct dentry *dentry, long offset)
@@ -507,19 +507,53 @@  static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
 	return vfs_setpos(file, offset, LONG_MAX);
 }
 
-static struct dentry *offset_find_next(struct offset_ctx *octx, loff_t offset)
+/* Cf. find_next_child() */
+static struct dentry *find_next_sibling_locked(struct dentry *dentry)
 {
-	MA_STATE(mas, &octx->mt, offset, offset);
+	struct dentry *found = NULL;
+
+	hlist_for_each_entry_from(dentry, d_sib) {
+		if (!simple_positive(dentry))
+			continue;
+		spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
+		if (simple_positive(dentry))
+			found = dget_dlock(dentry);
+		spin_unlock(&dentry->d_lock);
+		if (likely(found))
+			break;
+	}
+	return found;
+}
+
+static noinline_for_stack struct dentry *offset_dir_first(struct file *file)
+{
+	struct dentry *parent = file->f_path.dentry;
+	struct dentry *found;
+
+	spin_lock(&parent->d_lock);
+	found = find_next_sibling_locked(d_first_child(parent));
+	spin_unlock(&parent->d_lock);
+	return found;
+}
+
+static noinline_for_stack struct dentry *
+offset_dir_lookup(struct file *file, loff_t offset)
+{
+	struct dentry *parent = file->f_path.dentry;
 	struct dentry *child, *found = NULL;
+	struct inode *inode = d_inode(parent);
+	struct offset_ctx *octx = inode->i_op->get_offset_ctx(inode);
+
+	MA_STATE(mas, &octx->mt, offset, offset);
 
 	rcu_read_lock();
 	child = mas_find(&mas, LONG_MAX);
 	if (!child)
 		goto out;
-	spin_lock(&child->d_lock);
-	if (simple_positive(child))
-		found = dget_dlock(child);
-	spin_unlock(&child->d_lock);
+
+	spin_lock(&parent->d_lock);
+	found = find_next_sibling_locked(child);
+	spin_unlock(&parent->d_lock);
 out:
 	rcu_read_unlock();
 	return found;
@@ -534,29 +568,48 @@  static bool offset_dir_emit(struct dir_context *ctx, struct dentry *dentry)
 			  inode->i_ino, fs_umode_to_dtype(inode->i_mode));
 }
 
+static struct dentry *offset_dir_next(struct dentry *child)
+{
+	struct dentry *parent = child->d_parent;
+	struct dentry *found;
+
+	spin_lock(&parent->d_lock);
+	found = find_next_sibling_locked(d_next_sibling(child));
+	spin_unlock(&parent->d_lock);
+	return found;
+}
+
 static void offset_iterate_dir(struct file *file, struct dir_context *ctx)
 {
-	struct dentry *dir = file->f_path.dentry;
-	struct inode *inode = d_inode(dir);
-	struct offset_ctx *octx = inode->i_op->get_offset_ctx(inode);
-	struct dentry *dentry;
+	struct dentry *dentry, *next = NULL;
+
+	if (ctx->pos == DIR_OFFSET_FIRST)
+		dentry = offset_dir_first(file);
+	else
+		dentry = offset_dir_lookup(file, ctx->pos);
+	if (!dentry) {
+		/* ->private_data is protected by f_pos_lock */
+		offset_set_eod(file);
+		return;
+	}
 
 	while (true) {
-		dentry = offset_find_next(octx, ctx->pos);
-		if (!dentry) {
-			/* ->private_data is protected by f_pos_lock */
-			offset_set_eod(file);
-			return;
-		}
-
 		if (!offset_dir_emit(ctx, dentry)) {
-			dput(dentry);
+			ctx->pos = dentry2offset(dentry);
+			break;
+		}
+
+		next = offset_dir_next(dentry);
+		if (!next) {
+			offset_set_eod(file);
 			break;
 		}
 
-		ctx->pos = dentry2offset(dentry) + 1;
 		dput(dentry);
+		dentry = next;
 	}
+
+	dput(dentry);
 }
 
 /**