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

On Mon, 27 Apr 2026, Amir Goldstein wrote:
> On Mon, Apr 27, 2026 at 6:07 AM NeilBrown <[email protected]> wrote:
> >
> > From: NeilBrown <[email protected]>
> >
> > 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 <[email protected]>
> 
> Very nice!
> 
> One nit about the API
> 
> > ---
> >  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.
> 
> This contract is a bit subtle.
> We have plenty of contracts where the caller must dput() in case of success
> or in case of error, but must dput in case of a specific error that
> sounds fragile.
> 
> How about:
> * If the existing dentry is d_in_lookup(), d_alloc_noblock()
>  * returns with error %-EWOULDBLOCK and the blocking dentry is passed
>  * in @dentryp. Regardless of the returned error, if @dentryp is set by this
>  * function, the returned dentry must be dput() by the caller.

That is sensible, though I've used slightly different words.

Thanks,
NeilBrown

> 
> Thanks,
> Amir.
> 
> > + *
> > + * 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
> >
>