On Mon, Jan 04, 2021 at 02:24:38PM -0500, Andrea Arcangeli wrote: > On Mon, Jan 04, 2021 at 01:22:27PM +0100, Peter Zijlstra wrote: > > On Fri, Dec 25, 2020 at 01:25:28AM -0800, Nadav Amit wrote: > > > > > The scenario that happens in selftests/vm/userfaultfd is as follows: > > > > > > cpu0 cpu1 cpu2 > > > ---- ---- ---- > > > [ Writable PTE > > > cached in TLB ] > > > userfaultfd_writeprotect() > > > [ write-*unprotect* ] > > > mwriteprotect_range() > > > mmap_read_lock() > > > change_protection() > > > > > > change_protection_range() > > > ... > > > change_pte_range() > > > [ *clear* “write”-bit ] > > > [ defer TLB flushes ] > > > [ page-fault ] > > > ... > > > wp_page_copy() > > > cow_user_page() > > > [ copy page ] > > > [ write to old > > > page ] > > > ... > > > set_pte_at_notify() > > > > Yuck! > > > > Note, the above was posted before we figured out the details so it > wasn't showing the real deferred tlb flush that caused problems (the > one showed on the left causes zero issues). > > The problematic one not pictured is the one of the wrprotect that has > to be running in another CPU which is also isn't picture above. More > accurate traces are posted later in the thread. Lets assume CPU0 does a read-lock, W -> RO with deferred flush. > > Isn't this all rather similar to the problem that resulted in the > > tlb_flush_pending mess? > > > > I still think that's all fundamentally buggered, the much saner solution > > (IMO) would've been to make things wait for the pending flush, instead > > How do intend you wait in PT lock while the writer also has to take PT > lock repeatedly before it can do wake_up_var? > > If you release the PT lock before calling wait_tlb_flush_pending it > all falls apart again. I suppose you can check for pending, if found, release lock, wait for 0, and re-take the fault? > This I guess explains why a local pte/hugepmd smp local invlpg is the > only working solution for this issue, similarly to how it's done in rmap. In that case a local invalidate on CPU1 simply doesn't help anything. CPU1 needs to do a global invalidate or wait for the in-progress one to complete, such that CPU2 is sure to not have a W entry left before CPU1 goes and copies the page.
