Re: [PATCH 7/7] ovl: stop using lookup_one() in ovl_iterate().

[Amir Goldstein]

On Tue, Sep 29, 2026 at 5:43 AM NeilBrown <neilb@xxxxxxxxxxx> wrote:
>
> From: NeilBrown <neil@xxxxxxxxxx>
>
> lookup_one() is expected to be removed as it does not fit well with
> proposed changes to directory locking.
> Specifically d_alloc_parallel() will be ordered outside of i_rwsem
> and as iterate_shared() is called with i_rwsem held it is not safe
> to call d_alloc_parallel().
>
> We can instead call d_alloc_trylock() and then call the ->lookup, but
> that can fail if there is a lookup attempt concurrent with the
> readdir().
>
> ovl cannot afford for the lookup to fail as that could produce incorrect
> results, and it cannot safely drop i_rwsem temporarily as that could
> introduce races with handling of the directory cache.
>
> Instead we rely on the fact that ovl_iterate() has an exclusive lock on
> the directory, so any concurrent lookup will wait for the ovl_iterate()
> call to complete.  We allocate a separate dentry and if the lookup is
> successful, it is hashed with the result.

Please document this assumption with:

rwsem_assert_held_write(&dir->d_inode->i_rwsem);

and comment in the code. Having it in the git commit message is not
enough for future code reviewers.

Other than that, I have no further comments, but I would like
to have a review from Miklos before this patch lands, because
I don't remember him reviewing any of the revisions and this is subtle.
I am a bit concerned that we have no test coverage for this subtle code.

>
> When the concurrent lookup gets i_rwsem it mustn't do its own lookup -
> it must use the existing dentry.  This is found, if it exists, using
> try_lookup_noperm().
>
> Signed-off-by: NeilBrown <neil@xxxxxxxxxx>
> ---
>  fs/overlayfs/namei.c   | 12 ++++++++++++
>  fs/overlayfs/readdir.c | 26 ++++++++++++++++++++++++--
>  2 files changed, 36 insertions(+), 2 deletions(-)
>
> diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
> index ca899fdfaafd..f56e33fd4d6b 100644
> --- a/fs/overlayfs/namei.c
> +++ b/fs/overlayfs/namei.c
> @@ -1385,6 +1385,7 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
>         struct ovl_fs *ofs = OVL_FS(dentry->d_sb);
>         struct ovl_entry *poe = OVL_E(dentry->d_parent);
>         bool check_redirect = (ovl_redirect_follow(ofs) || ofs->numdatalayer);
> +       struct dentry *alias;
>         int err;
>         struct ovl_lookup_ctx ctx = {
>                 .dentry = dentry,
> @@ -1399,6 +1400,17 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
>         if (dentry->d_name.len > ofs->namelen)
>                 return ERR_PTR(-ENAMETOOLONG);
>
> +       /*
> +        * The existence 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/readdir.c b/fs/overlayfs/readdir.c
> index e7fe29cb6028..bc41da4bace8 100644
> --- a/fs/overlayfs/readdir.c
> +++ b/fs/overlayfs/readdir.c
> @@ -574,8 +574,30 @@ static int ovl_cache_update(const struct path *path, struct ovl_cache_entry *p,
>                 }
>         }
>         /* This checks also for xwhiteouts */
> -       this = lookup_one(mnt_idmap(path->mnt), &QSTR_LEN(p->name, p->len), dir);
> -       if (IS_ERR_OR_NULL(this) || !this->d_inode) {
> +       this = d_alloc_trylock(dir, &QSTR_LEN(p->name, p->len));
> +       if (this == ERR_PTR(-EWOULDBLOCK)) {
> +               /*
> +                * Some other thread is looking up this name and will
> +                * block on i_rwsem before it can complete the lookup.
> +                * We will do the lookup in a new dentry and when that
> +                * lookup gets a turn it will find and return this
> +                * dentry.
> +                */
> +               this = d_alloc_name(dir, p->name);
> +               if (!this)
> +                       this = ERR_PTR(-ENOMEM);
> +       }
> +       if (!IS_ERR(this) && d_unhashed(this)) {
> +               /* Either we got an in-lookup or we made our own unhashed */
> +               struct dentry *alias = ovl_lookup(dir->d_inode, this, 0);
> +
> +               d_lookup_done(this);
> +               if (alias) {
> +                       dput(this);
> +                       this = alias;
> +               }
> +       }
> +       if (IS_ERR(this) || !this->d_inode) {
>                 /* Mark a stale entry */
>                 p->is_whiteout = true;
>                 if (IS_ERR(this)) {

How do you test your patch set? Do you have tests that exercise
parallel create/lookup/readdir?

It would be irresponsible to merge code without showing that the `alias`
branches have been exercised. Do you have an idea how to do that?

Thanks,
Amir.