diff options
Diffstat (limited to 'fs/userfaultfd.c')
| -rw-r--r-- | fs/userfaultfd.c | 51 |
1 files changed, 25 insertions, 26 deletions
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index 97c4d71115d8..d80f94346199 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -396,32 +396,6 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason) goto out; /* - * If it's already released don't get it. This avoids to loop - * in __get_user_pages if userfaultfd_release waits on the - * caller of handle_userfault to release the mmap_lock. - */ - if (unlikely(READ_ONCE(ctx->released))) { - /* - * Don't return VM_FAULT_SIGBUS in this case, so a non - * cooperative manager can close the uffd after the - * last UFFDIO_COPY, without risking to trigger an - * involuntary SIGBUS if the process was starting the - * userfaultfd while the userfaultfd was still armed - * (but after the last UFFDIO_COPY). If the uffd - * wasn't already closed when the userfault reached - * this point, that would normally be solved by - * userfaultfd_must_wait returning 'false'. - * - * If we were to return VM_FAULT_SIGBUS here, the non - * cooperative manager would be instead forced to - * always call UFFDIO_UNREGISTER before it can safely - * close the uffd. - */ - ret = VM_FAULT_NOPAGE; - goto out; - } - - /* * Check that we can return VM_FAULT_RETRY. * * NOTE: it should become possible to return VM_FAULT_RETRY @@ -457,6 +431,31 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason) if (vmf->flags & FAULT_FLAG_RETRY_NOWAIT) goto out; + if (unlikely(READ_ONCE(ctx->released))) { + /* + * If a concurrent release is detected, do not return + * VM_FAULT_SIGBUS or VM_FAULT_NOPAGE, but instead always + * return VM_FAULT_RETRY with lock released proactively. + * + * If we were to return VM_FAULT_SIGBUS here, the non + * cooperative manager would be instead forced to + * always call UFFDIO_UNREGISTER before it can safely + * close the uffd, to avoid involuntary SIGBUS triggered. + * + * If we were to return VM_FAULT_NOPAGE, it would work for + * the fault path, in which the lock will be released + * later. However for GUP, faultin_page() does nothing + * special on NOPAGE, so GUP would spin retrying without + * releasing the mmap read lock, causing possible livelock. + * + * Here only VM_FAULT_RETRY would make sure the mmap lock + * be released immediately, so that the thread concurrently + * releasing the userfault would always make progress. + */ + release_fault_lock(vmf); + goto out; + } + /* take the reference before dropping the mmap_lock */ userfaultfd_ctx_get(ctx); |