Re: [RFC PATCH v3 0/9] Suppress negative dentry

[Miklos Szeredi <miklos@szeredi.hu>]

On Tue, May 19, 2020 at 11:24 AM cgxu <cgxu519@mykernel.net> wrote:
>
> On 5/19/20 4:21 PM, Miklos Szeredi wrote:
> > On Tue, May 19, 2020 at 7:02 AM cgxu <cgxu519@mykernel.net> wrote:
> >
> >> If we don't consider that only drop negative dentry of our lookup,
> >> it is possible to do like below, isn't it?
> > Yes, the code looks good, though I'd consider using d_lock on dentry
> > instead if i_lock on parent, something like this:
> >
> > if (d_is_negative(dentry) && dentry->d_lockref.count == 1) {
> >      spin_lock(&dentry->d_lock);
> >      /* Recheck condition under lock */
> >      if (d_is_negative(dentry) && dentry->d_lockref.count == 1)
> >          __d_drop(dentry)
> >      spin_unlock(&dentry->d_lock);
>
> And after this we will still treat 'dentry' as negative dentry and dput it
> regardless of the second check result of d_is_negative(dentry), right?

I'd restructure it in the same way as lookup_positive_unlocked()...

> > }
> >
> > But as Amir noted, we do need to take into account the case where
> > lower layers are shared by multiple overlays, in which case dropping
> > the negative dentries could result in a performance regression.
> > Have you looked at that case, and the effect of this patch on negative
> > dentry lookup performance?
>
> The container which is affected by this feature is just take advantage
> of previous another container but we could not guarantee that always
> happening. I think there no way for best of both worlds, consider that
> some malicious containers continuously make negative dentries by
> searching non-exist files, so that page cache of clean data, clean
> inodes/dentries will be freed by memory reclaim. All of those
> behaviors will impact the performance of other container instances.
>
> On the other hand, if this feature significantly affects particular
> container,
> doesn't that mean the container is noisy neighbor and should be restricted
> in some way?

Not necessarily.   Negative dentries can be useful and in case of
layers shared between two containers having negative dentries cached
in the lower layer can in theory positively affect performance.   I
don't have data to back this up, nor the opposite.  You should run
some numbers for container startup times with and without this patch.

Thanks,
Milklos