---- 在 星期一, 2020-05-18 15:52:48 Miklos Szeredi <miklos@xxxxxxxxxx> 撰写
----
> On Mon, May 18, 2020 at 7:27 AM Amir Goldstein <amir73il@xxxxxxxxx>
wrote:
> >
> > On Mon, May 18, 2020 at 3:53 AM Ian Kent <raven@xxxxxxxxxx> wrote:
> > >
> > > On Fri, 2020-05-15 at 15:20 +0800, Chengguang Xu wrote:
> > > > This series adds a new lookup flag LOOKUP_DONTCACHE_NEGATIVE
> > > > to indicate to drop negative dentry in slow path of lookup.
> > > >
> > > > In overlayfs, negative dentries in upper/lower layers are useless
> > > > after construction of overlayfs' own dentry, so in order to
> > > > effectively reclaim those dentries, specify
LOOKUP_DONTCACHE_NEGATIVE
> > > > flag when doing lookup in upper/lower layers.
> > >
> > > I've looked at this a couple of times now.
> > >
> > > I'm not at all sure of the wisdom of adding a flag to a VFS function
> > > that allows circumventing what a file system chooses to do.
> >
> > But it is not really a conscious choice is it?
> > How exactly does a filesystem express its desire to cache a negative
> > dentry? The documentation of lookup() in vfs.rst makes it clear that
> > it is not up to the filesystem to make that decision.
> > The VFS needs to cache the negative dentry on lookup(), so
> > it can turn it positive on create().
> > Low level kernel modules that call the VFS lookup() might know
> > that caching the negative dentry is counter productive.
> >
> > >
> > > I also do really see the need for it because only hashed negative
> > > dentrys will be retained by the VFS so, if you see a hashed negative
> > > dentry then you can cause it to be discarded on release of the last
> > > reference by dropping it.
> > >
> > > So what's different here, why is adding an argument to do that drop
> > > in the VFS itself needed instead of just doing it in overlayfs?
> >
> > That was v1 patch. It was dealing with the possible race of
> > returned negative dentry becoming positive before dropping it
> > in an intrusive manner.
> >
> > In retrospect, I think this race doesn't matter and there is no
> > harm in dropping a positive dentry in a race obviously caused by
> > accessing the underlying layer, which as documented results in
> > "undefined behavior".
> >
> > Miklos, am I missing something?
> > Dropping a positive dentry is harmful in case there's a long term
> reference to the dentry (e.g. an open file) since it will look as if
> the file was deleted, when in fact it wasn't.
> > It's possible to unhash a negative dentry in a safe way if we make
> sure it cannot become positive. One way is to grab d_lock and remove
> it from the hash table only if count is one.
> > So yes, we could have a helper to do that instead of the lookup flag.
> The disadvantage being that we'd also be dropping negatives that did
> not enter the cache because of our lookup.
>
If we don't consider that only drop negative dentry of our lookup,
it is possible to do like below, isn't it?
diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
index 723d17744758..fa339e23b0f8 100644
--- a/fs/overlayfs/namei.c
+++ b/fs/overlayfs/namei.c
@@ -200,7 +200,7 @@ static int ovl_lookup_single(struct dentry *base,
struct ovl_lookup_data *d,
int err;
bool last_element = !post[0];
- this = lookup_positive_unlocked(name, base, namelen);
+ this = lookup_one_len_unlocked(name, base, namelen);
if (IS_ERR(this)) {
err = PTR_ERR(this);
this = NULL;
@@ -209,6 +209,18 @@ static int ovl_lookup_single(struct dentry *base,
struct ovl_lookup_data *d,
goto out_err;
}
+ if (d_flags_negative(this->d_flags)) {
+ inode_lock_shared(base->d_inode);
+ if (d_flags_negative(this->d_flags))
+ d_drop(this);
+ inode_unlock_shared(base->d_inode);
+
+ dput(this);
+ this = NULL;
+ err = -ENOENT;
+ goto out;
+ }
+
if (ovl_dentry_weird(this)) {
/* Don't support traversing automounts and other
weirdness */
err = -EREMOTE;
Thanks,
cgxu
