On Wed, Dec 28, 2022 at 09:48:51PM +0900, Hyeonggon Yoo wrote: > Hello mm folks, > > I have a few questions about the current status of mmap_lock scalability. > > ============================================================= > What is currently causing the kernel to use mmap_lock to protect the maple tree? > ============================================================= > > I understand that the long-term goal is to remove the need for mmap_lock in readers > while traversing the maple tree, using techniques such as RCU or SPF. > What is the biggest obstacle preventing this from being achieved at this time? The long term goal is even larger than this. Ideally, the VMA tree would be protected by a spinlock rather than a mutex. That turned out to be too large a change for the moment (and isn't all that important compared to enabling RCU readers) > ================================================== > How does the maple tree provide RCU-safe manipulation of VMAs? > ================================================== > > Is it similar to the approach suggested in the RCUVM paper (replacing the original > root node with a new root node that shares most of its nodes and deferring > the freeing of stale nodes using RCU)? > > I'm having difficulty understanding the design of the maple tree in this regard. > > [RCUVM paper] https://pdos.csail.mit.edu/papers/rcuvm:asplos12.pdf While I've read the RCUVM paper, I wouldn't say it was particularly an inspiration. The Maple Tree is independent of the VM; it's a general purpose B-tree. As with any B-tree, when modifying a node, we don't touch nodes that we don't need to touch. As with any RCU data structure, we defer freeing things while RCU readers might still have a reference to them. We don't necessarily go all the way to the root node when modifying a leaf node. For example, if we have this structure: Root: Node A, 4000, Node B Node A: p1, 50, p2, 100, p3, 150, p4, 200, NULL, 250, p6, 1000, p7 Node B: p8, 4050, p9, 4100, p10, 4150, p11, 4200, NULL, 4250, p13 and we replace p4 with a NULL over the whole range from 150-199, we construct a new Node A2 that contains: Node A2: p1, 50, p2, 100, p3, 150, NULL, 250, p6, 1000, p7 and we simply write A2 over the entry in Root. Then we mark Node A as dead and RCU-free Node A. There's no need to replace Root as stores to a pointer are atomic. If we need to rebalance between Node A and Node B, we will need to create a new Root (as well as both A and B), mark all of them as dead and RCU-free them.
