On Thu, Dec 29, 2022 at 11:22:28PM +0900, Hyeonggon Yoo wrote: > On Wed, Dec 28, 2022 at 08:50:36PM +0000, Matthew Wilcox wrote: > > 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? The mmap_lock is taken for many, many things. So the plan was to have a spinlock in the maple tree (indeed, there's still one there; it's just in a union with the lockdep_map_p). VMA readers would walk the tree protected only by RCU; VMA writers would take the spinlock while modifying the tree. The work Suren, Liam & I are engaged in still uses the mmap semaphore for writers, but we do walk the tree under RCU protection. > > 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. Right, but it's a little more complex than that. The real "lock" on the VMA is actually a sequence count. https://lwn.net/Articles/906852/ does a good job of explaining it, but the VMA lock is really there as a convenient way for the writer to wait for readers to be sufficiently "finished" with handling the page fault that any conflicting changes will be correctly retired. https://www.infradead.org/~willy/linux/store-free-page-faults.html outlines how I intend to proceed from Suren's current scheme (where RCU is only used to protect the tree walk) to using RCU for the entire page fault.
