Adding few interested people in cc On Tue, Dec 03, 2019 at 02:21:47PM -0800, Matthew Wilcox wrote: > > [My thanks to Vlastimil, Michel, Liam, David, Davidlohr and Hugh for > their feedback on an earlier version of this. I think the solution > we discussed doesn't quite work, so here's one which I think does. > See the last two paragraphs in particular.] > > My preferred solution to the mmap_sem scalability problem is to allow > VMAs to be looked up under the RCU read lock then take a per-VMA lock. > I've been focusing on the first half of this problem (looking up VMAs > in an RCU-safe data structure) and ignoring the second half (taking a > lock while holding the RCU lock). > > We can't take a semaphore while holding the RCU lock in case we have to > sleep -- the VMA might not exist any more when we woke up. Making the > per-VMA lock a spinlock would be a massive change -- fault handlers are > currently called with the mmap_sem held and may sleep. So I think we > need a per-VMA refcount. That lets us sleep while handling a fault. > There are over 100 fault handlers in the kernel, and I don't want to > change the locking in all of them. > > That makes modifications to the tree a little tricky. At the moment, > we take the rwsem for write which waits for all readers to finish, then > we modify the VMAs, then we allow readers back in. With RCU, there is > no way to block readers, so different threads may (at the same time) > see both an old and a new VMA for the same virtual address. > > So calling mmap() looks like this: > > 1 allocate a new VMA > 2 update pointer(s) in maple tree > 3 sleep until old VMAs have a zero refcount > 4 synchronize_rcu() > 5 free old VMAs > 6 flush caches for affected range > 7 return to userspace > > While one thread is calling mmap(MAP_FIXED), two other threads which are > accessing the same address may see different data from each other and > have different page translations in their respective CPU caches until > the thread calling mmap() returns. I believe this is OK, but would > greatly appreciate hearing from people who know better. I do not believe this is OK, i believe this is wrong (not even considering possible hardware issues that can arise from such aliasing). That bein said i believe this can be solve "easily" when the new vma is added you mark it as a new born (VMA_BABY :)) and page fault will have to wait on it ie until the previous vma is fully gone and flush. So after step (6 flush caches) you remove the VMA_BABY flag before returning to userspace and page fault can resume. I would also mark old VMA with a ZOMBIE flag so that any reader might have a chance to back-off and retry. To check for that we should add a new check to vmf_insert_page() (and similar) to avoid inserting pfn in ZOMBIE vma. Note that i am not sure what we want to do here, can an application rely on rwsem serialization unknowingly ie could it have one thread doing page fault on a range that is about to be unmap by another thread ? I am not sure this can happen today without SEGFAULT thanks to serialization through rwsem. Anyway with BABY and ZOMBIE, it should behave mostly as it does today (modulo concurrency). > > Some people are concerned that a reference count on the VMA will lead to > contention moving from the mmap_sem to the refcount on a very large VMA > for workloads which have one giant VMA covering the entire working set. > For those workloads, I propose we use the existing ->map_pages() callback > (changed to return a vm_fault_t from the current void). > > It will be called with the RCU lock held and no reference count on > the vma. If it needs to sleep, it should bump the refcount, drop the > RCU lock, prepare enough so that the next call will not need to sleep, > then drop the refcount and return VM_FAULT_RETRY so the VM knows the > VMA is no longer good, and it needs to walk the VMA tree from the start. Just to make sure i understand, you propose that ->map_pages() becomes a new ->fault() handler that get calls before ->fault() without refcount so that we can update fs/drivers slowly to perform better in the new scheme (ie avoid the overead of refcounting if possible at all) ? The ->fault() callback would then be the "slow" path which will require a refcount on the vma (taken by core mm code before dropping rcu lock). Cheers, Jérôme
