> > > 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. > I see. My point was that the negative->positive transition cannot happen on underlying layers without user modifying underlying layers underneath overlay, so it is fine to be in the "undefined" behavior zone. > 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. > > I don't really care, both are probably good enough for the overlayfs case. > There is another point to consider. A negative underlying fs dentry may be useless for *this* overlayfs instance, but since lower layers can be shared among many overlayfs instances, for example, thousands of containers all testing for existence of file /etc/FOO on startup. It sounds like if we want to go through with DONTCACHE_NEGATIVE, that it should be opt-in behavior for overlayfs. Thanks, Amir.
