On 2/14/20 2:03 PM, Michel Lespinasse wrote: > Hi, > > I would like to propose this topic for LSF/MM 2020. This is a > continuation of discussions that were started at LSF/MM 2019 and have > informally continued since between the copied folks and I. > > The fact that mmap_sem locks the entire MM is causing a lot of > problems. The fundamental design hasn't changed in 20+ years, though a > number of hacks have been added (such as releasing the mmap_sem during > page faults) to work around the worst issues with it. In modern > threaded workloads, we often see multiple threads running > non-overlapping memory operations, which end up unnecessarily blocking > on each other because mmap_sem only supports locking the entire MM > rather than just the address range each thread is operating on. > > I have been working on a patch set to replace the mmap_sem rwsem with > a range lock, which should resolve this issue. This is currently > implemented through the page fault path and some very narrow cases of > mmap(); I am working to broaden the scope of the mmap changes before > sending this patch set publicly; I also know Davidlohr and Vlastimil > have been working on similar approaches in the past. JFTR you probably mean Davidlohr and Jan here from SUSE, not me, I'm relatively new to this topic :) But obviously interested in attending. Thanks for the proposal! > Another approach that is being explored is speculative page faults; I > know Peter and Laurent have been working on this in the past and > Matthew is giving this another look at the moment. I think this is a > different angle to approach the problem from; I think this solution is > not as generic (my understanding is that it only works for the page > fault path), but more efficient for the cases that it handles. > > I really would like to get a new discussion about this, to discuss the > concrete proposals that people have been working on and set a > direction moving forward. >
