+ userfaultfd-optimize-read-and-poll-to-be-o1-fix.patch added to -mm tree

+ userfaultfd-optimize-read-and-poll-to-be-o1-fix.patch added to -mm tree

[akpm]

The patch titled
     Subject: userfaultfd: fs/userfaultfd.c add more comments
has been added to the -mm tree.  Its filename is
     userfaultfd-optimize-read-and-poll-to-be-o1-fix.patch

This patch should soon appear at
    http://ozlabs.org/~akpm/mmots/broken-out/userfaultfd-optimize-read-and-poll-to-be-o1-fix.patch
and later at
    http://ozlabs.org/~akpm/mmotm/broken-out/userfaultfd-optimize-read-and-poll-to-be-o1-fix.patch

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------------------------------------------------------
From: Andrea Arcangeli <aarcange@redhat.com>
Subject: userfaultfd: fs/userfaultfd.c add more comments

Add more commentary.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 fs/userfaultfd.c |   13 +++++++++++++
 1 file changed, 13 insertions(+)

diff -puN fs/userfaultfd.c~userfaultfd-optimize-read-and-poll-to-be-o1-fix fs/userfaultfd.c
--- a/fs/userfaultfd.c~userfaultfd-optimize-read-and-poll-to-be-o1-fix
+++ a/fs/userfaultfd.c
@@ -456,6 +456,19 @@ static ssize_t userfaultfd_ctx_read(stru
 			 * Refile this userfault from
 			 * fault_pending_wqh to fault_wqh, it's not
 			 * pending anymore after we read it.
+			 *
+			 * Use list_del() by hand (as
+			 * userfaultfd_wake_function also uses
+			 * list_del_init() by hand) to be sure nobody
+			 * changes __remove_wait_queue() to use
+			 * list_del_init() in turn breaking the
+			 * !list_empty_careful() check in
+			 * handle_userfault(). The uwq->wq.task_list
+			 * must never be empty at any time during the
+			 * refile, or the waitqueue could disappear
+			 * from under us. The "wait_queue_head_t"
+			 * parameter of __remove_wait_queue() is unused
+			 * anyway.
 			 */
 			list_del(&uwq->wq.task_list);
 			__add_wait_queue(&ctx->fault_wqh, &uwq->wq);
_

Patches currently in -mm which might be from aarcange@redhat.com are

thp-cleanup-how-khugepaged-enters-freezer.patch
mm-drop-bogus-vm_bug_on_page-assert-in-put_page-codepath.patch
mm-avoid-tail-page-refcounting-on-non-thp-compound-pages.patch
userfaultfd-linux-documentation-vm-userfaultfdtxt.patch
userfaultfd-linux-documentation-vm-userfaultfdtxt-fix.patch
userfaultfd-waitqueue-add-nr-wake-parameter-to-__wake_up_locked_key.patch
userfaultfd-uapi.patch
userfaultfd-linux-userfaultfd_kh.patch
userfaultfd-add-vm_userfaultfd_ctx-to-the-vm_area_struct.patch
userfaultfd-add-vm_uffd_missing-and-vm_uffd_wp.patch
userfaultfd-call-handle_userfault-for-userfaultfd_missing-faults.patch
userfaultfd-teach-vma_merge-to-merge-across-vma-vm_userfaultfd_ctx.patch
userfaultfd-prevent-khugepaged-to-merge-if-userfaultfd-is-armed.patch
userfaultfd-add-new-syscall-to-provide-memory-externalization.patch
userfaultfd-add-new-syscall-to-provide-memory-externalization-fix.patch
userfaultfd-add-new-syscall-to-provide-memory-externalization-fix-fix.patch
userfaultfd-rename-uffd_apibits-into-features.patch
userfaultfd-rename-uffd_apibits-into-features-fixup.patch
userfaultfd-change-the-read-api-to-return-a-uffd_msg.patch
userfaultfd-change-the-read-api-to-return-a-uffd_msg-fix.patch
userfaultfd-wake-pending-userfaults.patch
userfaultfd-optimize-read-and-poll-to-be-o1.patch
userfaultfd-optimize-read-and-poll-to-be-o1-fix.patch
userfaultfd-allocate-the-userfaultfd_ctx-cacheline-aligned.patch
userfaultfd-solve-the-race-between-uffdio_copyzeropage-and-read.patch
userfaultfd-buildsystem-activation.patch
userfaultfd-activate-syscall.patch
userfaultfd-uffdio_copyuffdio_zeropage-uapi.patch
userfaultfd-mcopy_atomicmfill_zeropage-uffdio_copyuffdio_zeropage-preparation.patch
userfaultfd-avoid-mmap_sem-read-recursion-in-mcopy_atomic.patch
userfaultfd-uffdio_copy-and-uffdio_zeropage.patch
mm-oom_kill-remove-unnecessary-locking-in-oom_enable.patch
mm-oom_kill-clean-up-victim-marking-and-exiting-interfaces.patch
mm-oom_kill-switch-test-and-clear-of-known-tif_memdie-to-clear.patch
mm-oom_kill-generalize-oom-progress-waitqueue.patch
mm-oom_kill-remove-unnecessary-locking-in-exit_oom_victim.patch
mm-oom_kill-simplify-oom-killer-locking.patch
mm-page_alloc-inline-should_alloc_retry.patch
page-flags-trivial-cleanup-for-pagetrans-helpers.patch
page-flags-introduce-page-flags-policies-wrt-compound-pages.patch
page-flags-define-pg_locked-behavior-on-compound-pages.patch
page-flags-define-behavior-of-fs-io-related-flags-on-compound-pages.patch
page-flags-define-behavior-of-lru-related-flags-on-compound-pages.patch
page-flags-define-behavior-slb-related-flags-on-compound-pages.patch
page-flags-define-behavior-of-xen-related-flags-on-compound-pages.patch
page-flags-define-pg_reserved-behavior-on-compound-pages.patch
page-flags-define-pg_swapbacked-behavior-on-compound-pages.patch
page-flags-define-pg_swapcache-behavior-on-compound-pages.patch
page-flags-define-pg_mlocked-behavior-on-compound-pages.patch
page-flags-define-pg_uncached-behavior-on-compound-pages.patch
page-flags-define-pg_uptodate-behavior-on-compound-pages.patch
page-flags-look-on-head-page-if-the-flag-is-encoded-in-page-mapping.patch
mm-sanitize-page-mapping-for-tail-pages.patch