Messages in this thread | | | Date | Wed, 19 Jul 2023 14:16:31 -0700 | Subject | Re: Using userfaultfd with KVM's async page fault handling causes processes to hung waiting for mmap_lock to be released | From | Axel Rasmussen <> |
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Thanks for the detailed report Dimitris! I've CCed the MM mailing list and some folks who work on userfaultfd.
I took a look at this today, but I haven't quite come up with a solution.
I thought it might be as easy as changing userfaultfd_release() to set released *after* taking the lock. But no such luck, the ordering is what it is to deal with another subtle case:
WRITE_ONCE(ctx->released, true);
if (!mmget_not_zero(mm)) goto wakeup;
/* * Flush page faults out of all CPUs. NOTE: all page faults * must be retried without returning VM_FAULT_SIGBUS if * userfaultfd_ctx_get() succeeds but vma->vma_userfault_ctx * changes while handle_userfault released the mmap_lock. So * it's critical that released is set to true (above), before * taking the mmap_lock for writing. */ mmap_write_lock(mm);
I think perhaps the right thing to do is to have handle_userfault() release mmap_lock when it returns VM_FAULT_NOPAGE, and to have GUP deal with that appropriately? But, some investigation is required to be sure that's okay to do in the other non-GUP ways we can end up in handle_userfault().
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