[PATCH v3 10/19] VFS/ovl: add d_alloc_noblock_return()

NeilBrown
2026-04-27T04:07:22Z

ovl_lookup currently needs to check if a dentry with the same name has
already been added to the dcache as readdir might need to do. This
is an unnecessary performance cost to manage a rare race.

If ovl could know which in-lookup dentries raced with readdir, it could
limit the extra lookup to just those.

So add d_alloc_noblock_return() which provides the in-lookup dentry when
it returns -EWOULDBLOCK.

ovl_readdir() can use this this and flag the dentry such that
ovl_lookup() and easily check if a repeat lookup is needed.

Signed-off-by: NeilBrown <neil@brown.name>
---
fs/dcache.c | 50 ++++++++++++++++++++++++++++++++++++----
fs/overlayfs/namei.c | 23 ++++++++++--------
fs/overlayfs/overlayfs.h | 2 ++
fs/overlayfs/readdir.c | 7 ++++--
include/linux/dcache.h | 2 ++
5 files changed, 68 insertions(+), 16 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index a2ddfe811df3..2f11257b725b 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2749,7 +2749,8 @@ enum alloc_para {
static inline
struct dentry *__d_alloc_parallel(struct dentry *parent,
const struct qstr *name,
- enum alloc_para how)
+ enum alloc_para how,
+ struct dentry **dentryp)
{
unsigned int hash = name->hash;
struct hlist_bl_head *b = in_lookup_hash(parent, hash);
@@ -2836,7 +2837,10 @@ struct dentry *__d_alloc_parallel(struct dentry *parent,
case ALLOC_PARA_FAIL:
spin_unlock(&dentry->d_lock);
dput(new);
- dput(dentry);
+ if (dentryp)
+ *dentryp = dentry;
+ else
+ dput(dentry);
return ERR_PTR(-EWOULDBLOCK);
case ALLOC_PARA_WAIT:
wait_var_event_spinlock(&dentry->d_flags,
@@ -2899,7 +2903,7 @@ struct dentry *__d_alloc_parallel(struct dentry *parent,
struct dentry *d_alloc_parallel(struct dentry *parent,
const struct qstr *name)
{
- return __d_alloc_parallel(parent, name, ALLOC_PARA_WAIT);
+ return __d_alloc_parallel(parent, name, ALLOC_PARA_WAIT, NULL);
}
EXPORT_SYMBOL(d_alloc_parallel);

@@ -2931,11 +2935,49 @@ struct dentry *d_alloc_noblock(struct dentry *parent,

de = try_lookup_noperm(name, parent);
if (!de)
- de = __d_alloc_parallel(parent, name, ALLOC_PARA_FAIL);
+ de = __d_alloc_parallel(parent, name, ALLOC_PARA_FAIL, NULL);
return de;
}
EXPORT_SYMBOL(d_alloc_noblock);

+/**
+ * d_alloc_noblock_return() - find or allocate a new dentry
+ * @parent - dentry of the parent
+ * @name - name of the dentry within that parent.
+ * @dentryp - place to store the blocking dentry
+ *
+ * A new dentry is allocated and, providing it is unique, added to the
+ * relevant index.
+ * If an existing dentry is found with the same parent/name that is
+ * not d_in_lookup() then that is returned instead.
+ * If the existing dentry is d_in_lookup(), d_alloc_noblock()
+ * returns with error %-EWOULDBLOCK and the blocking dentry is passed
+ * in @dentryp. The dentry must be dput() by the caller.
+ *
+ * Thus if the returned dentry is d_in_lookup() then the caller has
+ * exclusive access until it completes the lookup.
+ * If the returned dentry is not d_in_lookup() then a lookup has
+ * already completed.
+ *
+ * The @name need not already have ->hash set.
+ *
+ * Returns: the dentry, whether found or allocated, or an error
+ * %-ENOMEM, %-EWOULDBLOCK, and anything returned by ->d_hash().
+ */
+struct dentry *d_alloc_noblock_return(struct dentry *parent,
+ struct qstr *name,
+ struct dentry **dentryp)
+{
+ struct dentry *de;
+
+ de = try_lookup_noperm(name, parent);
+ if (!de)
+ de = __d_alloc_parallel(parent, name, ALLOC_PARA_FAIL,
+ dentryp);
+ return de;
+}
+EXPORT_SYMBOL(d_alloc_noblock_return);
+
/*
* - Unhash the dentry
* - Retrieve and clear the waitqueue head in dentry
diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
index 69032eb2b1e2..524e661c3c8d 100644
--- a/fs/overlayfs/namei.c
+++ b/fs/overlayfs/namei.c
@@ -1400,16 +1400,19 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
if (dentry->d_name.len > ofs->namelen)
return ERR_PTR(-ENAMETOOLONG);

- /*
- * The existance of this in-lookup dentry might have forced
- * readdir to do the lookup with a new dentry. If so we must
- * return that one.
- */
- alias = try_lookup_noperm(&QSTR_LEN(dentry->d_name.name,
- dentry->d_name.len),
- dentry->d_parent);
- if (alias && !IS_ERR(alias))
- return alias;
+ if (ovl_dentry_test_flag(OVL_E_RACED_READDIR, dentry)) {
+ ovl_dentry_clear_flag(OVL_E_RACED_READDIR, dentry);
+ /*
+ * The existance of this in-lookup dentry might have
+ * forced readdir to do the lookup with a new dentry.
+ * If so we must return that one.
+ */
+ alias = try_lookup_noperm(&QSTR_LEN(dentry->d_name.name,
+ dentry->d_name.len),
+ dentry->d_parent);
+ if (alias && !IS_ERR(alias))
+ return alias;
+ }

with_ovl_creds(dentry->d_sb)
err = ovl_lookup_layers(&ctx, &d);
diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
index b75df37f70ac..bd6f1a25aca1 100644
--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -71,6 +71,8 @@ enum ovl_entry_flag {
OVL_E_CONNECTED,
/* Lower stack may contain xwhiteout entries */
OVL_E_XWHITEOUTS,
+ /* dentry was found in-lookup during readdir */
+ OVL_E_RACED_READDIR,
};

enum {
diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
index e03b32491941..e483bd939a8c 100644
--- a/fs/overlayfs/readdir.c
+++ b/fs/overlayfs/readdir.c
@@ -553,7 +553,7 @@ static int ovl_cache_update(const struct path *path, struct ovl_cache_entry *p,
{
struct dentry *dir = path->dentry;
struct ovl_fs *ofs = OVL_FS(dir->d_sb);
- struct dentry *this = NULL;
+ struct dentry *this = NULL, *in_lookup;
enum ovl_path_type type;
u64 ino = p->real_ino;
int xinobits = ovl_xino_bits(ofs);
@@ -574,7 +574,8 @@ static int ovl_cache_update(const struct path *path, struct ovl_cache_entry *p,
}
}
/* This checks also for xwhiteouts */
- this = d_alloc_noblock(dir, &QSTR_LEN(p->name, p->len));
+ this = d_alloc_noblock_return(dir, &QSTR_LEN(p->name, p->len),
+ &in_lookup);
if (this == ERR_PTR(-EWOULDBLOCK)) {
/*
* Some other thead is looking up this name and will
@@ -583,6 +584,8 @@ static int ovl_cache_update(const struct path *path, struct ovl_cache_entry *p,
* lookup gets a turn it will find and return this
* dentry.
*/
+ ovl_dentry_set_flag(OVL_E_RACED_READDIR, in_lookup);
+ dput(in_lookup);
this = d_alloc_name(dir, p->name);
}
if (!IS_ERR(this) && !d_unhashed(this)) {
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index 662b98185337..db7cdcbac775 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -258,6 +258,8 @@ extern struct dentry * d_alloc(struct dentry *, const struct qstr *);
extern struct dentry * d_alloc_anon(struct super_block *);
extern struct dentry * d_alloc_parallel(struct dentry *, const struct qstr *);
extern struct dentry * d_alloc_noblock(struct dentry *, struct qstr *);
+extern struct dentry * d_alloc_noblock_return(struct dentry *, struct qstr *,
+ struct dentry **);
extern struct dentry * d_splice_alias(struct inode *, struct dentry *);
struct dentry *d_duplicate(struct dentry *dentry);
/* weird procfs mess; *NOT* exported */
--
2.50.0.107.gf914562f5916.dirty