On Mon, May 18, 2020 at 10:52 AM Amir Goldstein <amir73il@xxxxxxxxx> wrote: > > > > > 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. Right, I don't think you can actually crash a filesystem by unhashing a positive dentry in the middle of a create op, but it would definitely be prudent to avoid that. > > > 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. Good point. Thanks, Miklos
