> On Dec 21, 2020, at 12:52 PM, Peter Xu <peterx@xxxxxxxxxx> wrote: > > On Mon, Dec 21, 2020 at 07:51:52PM +0000, Nadav Amit wrote: >>> On Dec 21, 2020, at 11:28 AM, Peter Xu <peterx@xxxxxxxxxx> wrote: >>> >>> On Sat, Nov 28, 2020 at 04:45:38PM -0800, Nadav Amit wrote: >>>> From: Nadav Amit <namit@xxxxxxxxxx> >>>> >>>> When userfaultfd copy-ioctl fails since the PTE already exists, an >>>> -EEXIST error is returned and the faulting thread is not woken. The >>>> current userfaultfd test does not wake the faulting thread in such case. >>>> The assumption is presumably that another thread set the PTE through >>>> copy/wp ioctl and would wake the faulting thread or that alternatively >>>> the fault handler would realize there is no need to "must_wait" and >>>> continue. This is not necessarily true. >>>> >>>> There is an assumption that the "must_wait" tests in handle_userfault() >>>> are sufficient to provide definitive answer whether the offending PTE is >>>> populated or not. However, userfaultfd_must_wait() test is lockless. >>>> Consequently, concurrent calls to ptep_modify_prot_start(), for >>>> instance, can clear the PTE and can cause userfaultfd_must_wait() >>>> to wrongly assume it is not populated and a wait is needed. >>> >>> Yes userfaultfd_must_wait() is lockless, however my understanding is that we'll >>> enqueue before reading the page table, which seems to me that we'll always get >>> notified even the race happens. Should apply to either UFFDIO_WRITEPROTECT or >>> UFFDIO_COPY, iiuc, as long as we follow the order of (1) modify pgtable (2) >>> wake sleeping threads. Then it also means that when must_wait() returned true, >>> it should always get waked up when fault resolved. >>> >>> Taking UFFDIO_COPY as example, even if UFFDIO_COPY happen right before >>> must_wait() calls: >>> >>> worker thread uffd thread >>> ------------- ----------- >>> >>> handle_userfault >>> spin_lock(fault_pending_wqh) >>> enqueue() >>> set_current_state(INTERRUPTIBLE) >>> spin_unlock(fault_pending_wqh) >>> must_wait() >>> lockless walk page table >>> UFFDIO_COPY >>> fill in the hole >>> wake up threads >>> (this will wake up worker thread too?) >>> schedule() >>> (which may return immediately?) >>> >>> While here fault_pending_wqh is lock protected. I just feel like there's some >>> other reason to cause the thread to stall. Or did I miss something? >> >> But what happens if the copy completed before the enqueuing? Assume >> the page is write-protected during UFFDIO_COPY: >> >> >> cpu0 cpu1 >> ---- ---- >> handle_userfault >> UFFDIO_COPY >> [ write-protected ] >> fill in the hole >> wake up threads >> [nothing to wake] >> >> UFFD_WP (unprotect) >> logically marks as unprotected >> [nothing to wake] >> >> spin_lock(fault_pending_wqh) >> enqueue() >> set_current_state(INTERRUPTIBLE) >> spin_unlock(fault_pending_wqh) >> must_wait() >> >> [ #PF on the same PTE >> due to write-protection ] >> >> ... >> wp_page_copy() >> ptep_clear_flush_notify() >> [ PTE is clear ] >> >> lockless walk page table >> pte_none(*pte) -> must wait >> >> Note that additional scenarios are possible. For instance, instead of >> wp_page_copy(), we can have other change_pte_range() (due to worker’s >> mprotect() or NUMA balancing), calling ptep_modify_prot_start() and clearing >> the PTE. >> >> Am I missing something? > > Ah I see your point, thanks. I think you're right: > > Reviewed-by: Peter Xu <peterx@xxxxxxxxxx> > > Would you mind adding something like above into the commit message if you're > going to repost? IMHO it would even be nicer to mention why > UFFDIO_WRITEPROTECT does not need this extra wakeup (I think it's because it'll > do the wakeup unconditionally anyway). Yes, the commit log needs to be fixed. I will update it based on your feedback on RFC-v2. Thanks, Nadav
