In order to walk the page tables without the mmap semaphore, it must be possible to prevent them from being freed and reused (eg if munmap() races with viewing /proc/$pid/smaps). There is various commentary within the mm on how to prevent this. One way is to disable interrupts, relying on that to block rcu_sched or IPIs. I don't think the RT people are terribly happy about reading a proc file disabling interrupts, and it doesn't work for architectures that free page tables directly instead of batching them into an rcu_sched (because the IPI may not be sent to this CPU if the task has never run on it). See "Fast GUP" in mm/gup.c Ideally, I'd like rcu_read_lock() to delay page table reuse. This is close to trivial for architectures which use entire pages or multiple pages for levels of their page tables as we can use the rcu_head embedded in struct page to queue the page for RCU. s390 and powerpc are the only two architectures I know of that have levels of their page table that are smaller than their PAGE_SIZE. I'd like to discuss options. There may be a complicated scheme that allows partial pages to be freed via RCU, but I have something simpler in mind. For powerpc in particular, it can have a PAGE_SIZE of 64kB and then the MMU wants to see 4kB entries in the PMD. I suggest that instead of allocating each 4kB entry individually, we allocate a 64kB page and fill in 16 consecutive PMDs. This could cost a bit more memory (although if you've asked for a CONFIG_PAGE_SIZE of 64kB, you presumably don't care too much about it), but it'll make future page faults cheaper (as the PMDs will already be present, assuming you have good locality of reference). I'd like to hear better ideas than this.
