[PATCH 0/2] userfaultfdv4.1 update for -mm

[PATCH 0/2] userfaultfdv4.1 update for -mm

[Andrea Arcangeli]

Hello,

Here some change I already folded earlier, extracted in order to apply
cleanly at the end of current -mm.

To fold the userfaultfd.txt update the part in "userfaultfd: change
the read API to return a uffd_msg" should also be extracted and folded
first or this one will reject.

Andrea Arcangeli (2):
  userfaultfd: documentation update
  userfaultfd: fs/userfaultfd.c add more comments

 Documentation/vm/userfaultfd.txt | 16 +++++++++-------
 fs/userfaultfd.c                 | 28 +++++++++++++++++++++++++++-
 2 files changed, 36 insertions(+), 8 deletions(-)

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[PATCH 1/2] userfaultfd: documentation update

[Andrea Arcangeli]

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
---
 Documentation/vm/userfaultfd.txt | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/Documentation/vm/userfaultfd.txt b/Documentation/vm/userfaultfd.txt
index 3557edd..70a3c94 100644
--- a/Documentation/vm/userfaultfd.txt
+++ b/Documentation/vm/userfaultfd.txt
@@ -3,8 +3,8 @@
 == Objective ==
 
 Userfaults allow the implementation of on-demand paging from userland
-and more generally they allow userland to take control various memory
-page faults, something otherwise only the kernel code could do.
+and more generally they allow userland to take control of various
+memory page faults, something otherwise only the kernel code could do.
 
 For example userfaults allows a proper and more optimal implementation
 of the PROT_NONE+SIGSEGV trick.
@@ -47,10 +47,10 @@ When first opened the userfaultfd must be enabled invoking the
 UFFDIO_API ioctl specifying a uffdio_api.api value set to UFFD_API (or
 a later API version) which will specify the read/POLLIN protocol
 userland intends to speak on the UFFD and the uffdio_api.features
-userland needs to be enabled. The UFFDIO_API ioctl if successful
-(i.e. if the requested uffdio_api.api is spoken also by the running
-kernel and the requested features are going to be enabled) will return
-into uffdio_api.features and uffdio_api.ioctls two 64bit bitmasks of
+userland requires. The UFFDIO_API ioctl if successful (i.e. if the
+requested uffdio_api.api is spoken also by the running kernel and the
+requested features are going to be enabled) will return into
+uffdio_api.features and uffdio_api.ioctls two 64bit bitmasks of
 respectively all the available features of the read(2) protocol and
 the generic ioctl available.
 
@@ -77,7 +77,9 @@ The primary ioctl to resolve userfaults is UFFDIO_COPY. That
 atomically copies a page into the userfault registered range and wakes
 up the blocked userfaults (unless uffdio_copy.mode &
 UFFDIO_COPY_MODE_DONTWAKE is set). Other ioctl works similarly to
-UFFDIO_COPY.
+UFFDIO_COPY. They're atomic as in guaranteeing that nothing can see an
+half copied page since it'll keep userfaulting until the copy has
+finished.
 
 == QEMU/KVM ==
 

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[PATCH 2/2] userfaultfd: fs/userfaultfd.c add more comments

[Andrea Arcangeli]

Add more commentary.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
---
 fs/userfaultfd.c | 28 +++++++++++++++++++++++++++-
 1 file changed, 27 insertions(+), 1 deletion(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index f601f27..a519f74 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -86,7 +86,20 @@ static int userfaultfd_wake_function(wait_queue_t *wq, unsigned mode,
 		goto out;
 	ret = wake_up_state(wq->private, mode);
 	if (ret)
-		/* wake only once, autoremove behavior */
+		/*
+		 * Wake only once, autoremove behavior.
+		 *
+		 * After the effect of list_del_init is visible to the
+		 * other CPUs, the waitqueue may disappear from under
+		 * us, see the !list_empty_careful() in
+		 * handle_userfault(). try_to_wake_up() has an
+		 * implicit smp_mb__before_spinlock, and the
+		 * wq->private is read before calling the extern
+		 * function "wake_up_state" (which in turns calls
+		 * try_to_wake_up). While the spin_lock;spin_unlock;
+		 * wouldn't be enough, the smp_mb__before_spinlock is
+		 * enough to avoid an explicit smp_mb() here.
+		 */
 		list_del_init(&wq->task_list);
 out:
 	return ret;
@@ -511,6 +524,19 @@ static ssize_t userfaultfd_ctx_read(struct userfaultfd_ctx *ctx, int no_wait,
 			 * 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);

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