On Wed, Dec 28, 2022 at 08:50:36PM +0000, Matthew Wilcox wrote: > 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. You mean replacing mmap_lock rwsem with a spinlock? How is that possible if readers can take it for page fault? > That turned out > to be too large a change for the moment (and isn't all that important > compared to enabling RCU readers) Yeah, better to take one step at a time. > > > ================================================== > > 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. My intention was to ask how to synchronize with other VMA operations after the tree traversal with RCU. (Because it's unreasonable to handle page fault in RCU read-side critical section) Per-VMA lock seem to solve it by taking the VMA lock in read mode within RCU read-side critical section. > 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. Thank you for explaining things in an easy and intuitive way. Okay, I get it's not a big problem to update the value(s) in a B-tree in RCU-safe way. > 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. -- Thanks, Hyeonggon
