On Mon, 2018-07-02 at 14:18 -0700, Andrew Morton wrote: > On Mon, 2 Jul 2018 12:34:00 -0700 Linus Torvalds <torvalds@linux-foun > dation.org> wrote: > > > On Sun, Jul 1, 2018 at 10:52 PM Waiman Long <longman@xxxxxxxxxx> > > wrote: > > > > > > A rogue application can potentially create a large number of > > > negative > > > dentries in the system consuming most of the memory available if > > > it > > > is not under the direct control of a memory controller that > > > enforce > > > kernel memory limit. > > > > I certainly don't mind the patch series, but I would like it to be > > accompanied with some actual example numbers, just to make it all a > > bit more concrete. > > > > Maybe even performance numbers showing "look, I've filled the > > dentry > > lists with nasty negative dentries, now it's all slower because we > > walk those less interesting entries". > > > > (Please cc linux-mm@xxxxxxxxx on this work) > > Yup. The description of the user-visible impact of current behavior > is far too vague. > > In the [5/6] changelog it is mentioned that a large number of -ve > dentries can lead to oom-killings. This sounds bad - -ve dentries > should be trivially reclaimable and we shouldn't be oom-killing in > such a situation. If you're old enough, it's déjà vu; Andrea went on a negative dentry rampage about 15 years ago: https://lkml.org/lkml/2002/5/24/71 I think the summary of the thread is that it's not worth it because dentries are a clean cache, so they're immediately shrinkable. > Dumb question: do we know that negative dentries are actually > worthwhile? Has anyone checked in the past couple of > decades? Perhaps our lookups are so whizzy nowadays that we don't > need them? There are still a lot of applications that keep looking up non-existent files, so I think it's still beneficial to keep them. Apparently apache still looks for a .htaccess file in every directory it traverses, for instance. Round tripping every one of these to disk instead of caching it as a negative dentry would seem to be a performance loser here. However, actually measuring this again might be useful. James
