[RFC PATCH v3 1/9] fs/dcache: Introduce a new lookup flag LOOKUP_DONTCACHE_NEGATIVE

[Chengguang Xu <cgxu519@mykernel.net>]

Introduce a new lookup flag LOOKUP_DONTCACHE_NEGATIVE to skip
caching negative dentry in lookup.

Signed-off-by: Chengguang Xu <cgxu519@mykernel.net>
---
 fs/namei.c            | 14 ++++++++++----
 include/linux/namei.h |  9 +++++++--
 2 files changed, 17 insertions(+), 6 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index a320371899cf..a258f0a1d5d6 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1532,6 +1532,9 @@ static struct dentry *__lookup_slow(const struct qstr *name,
 		if (unlikely(old)) {
 			dput(dentry);
 			dentry = old;
+		} else if ((flags & LOOKUP_DONTCACHE_NEGATIVE) &&
+			   d_is_negative(dentry)) {
+			d_drop(dentry);
 		}
 	}
 	return dentry;
@@ -2554,6 +2557,7 @@ EXPORT_SYMBOL(lookup_one_len);
  * @name:	pathname component to lookup
  * @base:	base directory to lookup from
  * @len:	maximum length @len should be interpreted to
+ * @flags:	lookup flags
  *
  * Note that this routine is purely a helper for filesystem usage and should
  * not be called by generic code.
@@ -2562,7 +2566,8 @@ EXPORT_SYMBOL(lookup_one_len);
  * i_mutex held, and will take the i_mutex itself if necessary.
  */
 struct dentry *lookup_one_len_unlocked(const char *name,
-				       struct dentry *base, int len)
+				       struct dentry *base, int len,
+				       unsigned int flags)
 {
 	struct qstr this;
 	int err;
@@ -2574,7 +2579,7 @@ struct dentry *lookup_one_len_unlocked(const char *name,
 
 	ret = lookup_dcache(&this, base, 0);
 	if (!ret)
-		ret = lookup_slow(&this, base, 0);
+		ret = lookup_slow(&this, base, flags);
 	return ret;
 }
 EXPORT_SYMBOL(lookup_one_len_unlocked);
@@ -2588,9 +2593,10 @@ EXPORT_SYMBOL(lookup_one_len_unlocked);
  * this one avoids such problems.
  */
 struct dentry *lookup_positive_unlocked(const char *name,
-				       struct dentry *base, int len)
+				       struct dentry *base, int len,
+				       unsigned int flags)
 {
-	struct dentry *ret = lookup_one_len_unlocked(name, base, len);
+	struct dentry *ret = lookup_one_len_unlocked(name, base, len, flags);
 	if (!IS_ERR(ret) && d_flags_negative(smp_load_acquire(&ret->d_flags))) {
 		dput(ret);
 		ret = ERR_PTR(-ENOENT);
diff --git a/include/linux/namei.h b/include/linux/namei.h
index a4bb992623c4..1d43fc481e47 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -49,6 +49,9 @@ enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT};
 /* LOOKUP_* flags which do scope-related checks based on the dirfd. */
 #define LOOKUP_IS_SCOPED (LOOKUP_BENEATH | LOOKUP_IN_ROOT)
 
+/* Hint: don't cache negative. */
+#define LOOKUP_DONTCACHE_NEGATIVE	0x200000
+
 extern int path_pts(struct path *path);
 
 extern int user_path_at_empty(int, const char __user *, unsigned, struct path *, int *empty);
@@ -68,8 +71,10 @@ extern struct dentry *kern_path_locked(const char *, struct path *);
 
 extern struct dentry *try_lookup_one_len(const char *, struct dentry *, int);
 extern struct dentry *lookup_one_len(const char *, struct dentry *, int);
-extern struct dentry *lookup_one_len_unlocked(const char *, struct dentry *, int);
-extern struct dentry *lookup_positive_unlocked(const char *, struct dentry *, int);
+extern struct dentry *lookup_one_len_unlocked(const char *name,
+			struct dentry *base, int len, unsigned int flags);
+extern struct dentry *lookup_positive_unlocked(const char *name,
+			struct dentry *base, int len, unsigned int flags);
 
 extern int follow_down_one(struct path *);
 extern int follow_down(struct path *);
-- 
2.20.1