[PATCH RFC 4/4] fs: try an opportunistic lookup for O_CREAT opens too

[Jeff Layton <jlayton@xxxxxxxxxx>]

Today, when opening a file we'll typically do a fast lookup, but if
O_CREAT is set, the kernel always takes the exclusive inode lock. I'm
sure this was done with the expectation that O_CREAT being set means
that we expect to do the create, but that's often not the case. Many
programs set O_CREAT even in scenarios where the file already exists.

This patch rearranges the pathwalk-for-open code to also attempt a
fast_lookup in the O_CREAT case.  Have the code always do a fast_lookup
(unless O_EXCL is set), and return that without taking the inode_lock
when a positive dentry is found in the O_CREAT codepath.

Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
---
 fs/namei.c | 43 ++++++++++++++++++++++++++++++++++++-------
 1 file changed, 36 insertions(+), 7 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index b9bdb8e6214a..1793ed090314 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -3538,7 +3538,7 @@ static const char *open_last_lookups(struct nameidata *nd,
 	struct dentry *dir = nd->path.dentry;
 	int open_flag = op->open_flag;
 	bool got_write = false;
-	struct dentry *dentry;
+	struct dentry *dentry = NULL;
 	const char *res;
 
 	nd->flags |= op->intent;
@@ -3549,28 +3549,57 @@ static const char *open_last_lookups(struct nameidata *nd,
 		return handle_dots(nd, nd->last_type);
 	}
 
-	if (!(open_flag & O_CREAT)) {
-		if (nd->last.name[nd->last.len])
+	/*
+	 * We _can_ be in RCU mode here. For everything but O_EXCL case, do a
+	 * fast lookup for the dentry first. For O_CREAT case, we are only
+	 * interested in positive dentries. If nothing suitable is found,
+	 * fall back to locked codepath.
+	 */
+	if ((open_flag & (O_CREAT | O_EXCL)) != (O_CREAT | O_EXCL)) {
+		/* Trailing slashes? */
+		if (unlikely(nd->last.name[nd->last.len]))
 			nd->flags |= LOOKUP_FOLLOW | LOOKUP_DIRECTORY;
-		/* we _can_ be in RCU mode here */
+
 		dentry = lookup_fast(nd);
 		if (IS_ERR(dentry))
 			return ERR_CAST(dentry);
+	}
+
+	if (!(open_flag & O_CREAT)) {
 		if (likely(dentry))
 			goto finish_lookup;
 
 		if (WARN_ON_ONCE(nd->flags & LOOKUP_RCU))
 			return ERR_PTR(-ECHILD);
 	} else {
-		/* create side of things */
+		/* If negative dentry was found earlier,
+		 * discard it as we'll need to use the slow path anyway.
+		 */
 		if (nd->flags & LOOKUP_RCU) {
-			if (!try_to_unlazy(nd))
+			bool unlazied;
+
+			/* discard negative dentry if one was found */
+			if (dentry && !dentry->d_inode)
+				dentry = NULL;
+
+			unlazied = dentry ? try_to_unlazy_next(nd, dentry) :
+					    try_to_unlazy(nd);
+			if (!unlazied)
 				return ERR_PTR(-ECHILD);
+		} else if (dentry && !dentry->d_inode) {
+			/* discard negative dentry if one was found */
+			dput(dentry);
+			dentry = NULL;
 		}
 		audit_inode(nd->name, dir, AUDIT_INODE_PARENT);
+
 		/* trailing slashes? */
-		if (unlikely(nd->last.name[nd->last.len]))
+		if (unlikely(nd->last.name[nd->last.len])) {
+			dput(dentry);
 			return ERR_PTR(-EISDIR);
+		}
+		if (dentry)
+			goto finish_lookup;
 	}
 
 	if (open_flag & (O_CREAT | O_TRUNC | O_WRONLY | O_RDWR)) {

-- 
2.45.2