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. 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. -- Michel "Walken" Lespinasse A program is never fully debugged until the last user dies.
