On 07/03/2018 07:19 AM, Andrew Morton wrote: > On Mon, 02 Jul 2018 15:34:40 -0700 James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> wrote: > >> 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 > That's kinda funny. > >> 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. > Yes, "should be". I could understand that the presence of huge > nunmbers of -ve dentries could result in undesirable reclaim of > pagecache, etc. Triggering oom-killings is very bad, and presumably > has the same cause. > > Before we go and add a large amount of code to do the shrinker's job > for it, we should get a full understanding of what's going wrong. Is > it because the dentry_lru had a mixture of +ve and -ve dentries? > Should we have a separate LRU for -ve dentries? Are we appropriately > aging the various dentries? etc. I have actually investigated having a separate LRU for negative dentries. That will result in a far more invasive patch that will be more disruptive. Another change that was suggested by a colleague is to put a newly created -ve dentry to the tail (LRU end) of the LRU and move it to the head only if it is accessed a second time. That will put most of the negative dentries at the tail that will be more easily trimmed away. Cheers, Longman
