On Tue, Dec 03, 2019 at 02:21:47PM -0800, Matthew Wilcox wrote: > 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). Do you see this approach to be regression-free for uncontended case? I doubt it will not cause regressions for signle-threaded applications... > We currently only have one ->map_pages() callback, and it's > filemap_map_pages(). It only needs to sleep in one place -- to allocate > a PTE table. I think that can be allocated ahead of time if needed. No, filemap_map_pages() doesn't sleep. It cannot. Whole body of the function is under rcu_read_lock(). It uses pre-allocated page table. See do_fault_around(). -- Kirill A. Shutemov
