public inbox for [email protected]
 help / color / mirror / Atom feed
From: Nadav Amit <[email protected]>
To: [email protected]
Cc: Nadav Amit <[email protected]>, Jens Axboe <[email protected]>,
	Andrea Arcangeli <[email protected]>,
	Peter Xu <[email protected]>,
	Alexander Viro <[email protected]>,
	[email protected], [email protected],
	[email protected]
Subject: [RFC PATCH 03/13] selftests/vm/userfaultfd: wake after copy failure
Date: Sat, 28 Nov 2020 16:45:38 -0800	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>

From: Nadav Amit <[email protected]>

When userfaultfd copy-ioctl fails since the PTE already exists, an
-EEXIST error is returned and the faulting thread is not woken. The
current userfaultfd test does not wake the faulting thread in such case.
The assumption is presumably that another thread set the PTE through
copy/wp ioctl and would wake the faulting thread or that alternatively
the fault handler would realize there is no need to "must_wait" and
continue. This is not necessarily true.

There is an assumption that the "must_wait" tests in handle_userfault()
are sufficient to provide definitive answer whether the offending PTE is
populated or not. However, userfaultfd_must_wait() test is lockless.
Consequently, concurrent calls to ptep_modify_prot_start(), for
instance, can clear the PTE and can cause userfaultfd_must_wait()
to wrongly assume it is not populated and a wait is needed.

There are therefore 3 options:
(1) Change the tests to wake on copy failure.
(2) Wake faulting thread unconditionally on zero/copy ioctls before
    returning -EEXIST.
(3) Change the userfaultfd_must_wait() to hold locks.

This patch took the first approach, but the others are valid solutions
with different tradeoffs.

Cc: Jens Axboe <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Nadav Amit <[email protected]>
---
 tools/testing/selftests/vm/userfaultfd.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/tools/testing/selftests/vm/userfaultfd.c b/tools/testing/selftests/vm/userfaultfd.c
index 9b0912a01777..f7e6cf43db71 100644
--- a/tools/testing/selftests/vm/userfaultfd.c
+++ b/tools/testing/selftests/vm/userfaultfd.c
@@ -484,6 +484,18 @@ static void retry_copy_page(int ufd, struct uffdio_copy *uffdio_copy,
 	}
 }
 
+static void wake_range(int ufd, unsigned long addr, unsigned long len)
+{
+	struct uffdio_range uffdio_wake;
+
+	uffdio_wake.start = addr;
+	uffdio_wake.len = len;
+
+	if (ioctl(ufd, UFFDIO_WAKE, &uffdio_wake))
+		fprintf(stderr, "error waking %lu\n",
+			addr), exit(1);
+}
+
 static int __copy_page(int ufd, unsigned long offset, bool retry)
 {
 	struct uffdio_copy uffdio_copy;
@@ -507,6 +519,7 @@ static int __copy_page(int ufd, unsigned long offset, bool retry)
 				uffdio_copy.copy);
 			exit(1);
 		}
+		wake_range(ufd, uffdio_copy.dst, page_size);
 	} else if (uffdio_copy.copy != page_size) {
 		fprintf(stderr, "UFFDIO_COPY unexpected copy %Ld\n",
 			uffdio_copy.copy); exit(1);
-- 
2.25.1


  parent reply	other threads:[~2020-11-29  0:50 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-29  0:45 [RFC PATCH 00/13] fs/userfaultfd: support iouring and polling Nadav Amit
2020-11-29  0:45 ` [RFC PATCH 01/13] fs/userfaultfd: fix wrong error code on WP & !VM_MAYWRITE Nadav Amit
2020-12-01 21:22   ` Mike Kravetz
2020-12-21 19:01     ` Peter Xu
2020-11-29  0:45 ` [RFC PATCH 02/13] fs/userfaultfd: fix wrong file usage with iouring Nadav Amit
2020-11-29  0:45 ` Nadav Amit [this message]
2020-12-21 19:28   ` [RFC PATCH 03/13] selftests/vm/userfaultfd: wake after copy failure Peter Xu
2020-12-21 19:51     ` Nadav Amit
2020-12-21 20:52       ` Peter Xu
2020-12-21 20:54         ` Nadav Amit
2020-11-29  0:45 ` [RFC PATCH 04/13] fs/userfaultfd: simplify locks in userfaultfd_ctx_read Nadav Amit
2020-11-29  0:45 ` [RFC PATCH 05/13] fs/userfaultfd: introduce UFFD_FEATURE_POLL Nadav Amit
2020-11-29  0:45 ` [RFC PATCH 06/13] iov_iter: support atomic copy_page_from_iter_iovec() Nadav Amit
2020-11-29  0:45 ` [RFC PATCH 07/13] fs/userfaultfd: support read_iter to use io_uring Nadav Amit
2020-11-30 18:20   ` Jens Axboe
2020-11-30 19:23     ` Nadav Amit
2020-11-29  0:45 ` [RFC PATCH 08/13] fs/userfaultfd: complete reads asynchronously Nadav Amit
2020-11-29  0:45 ` [RFC PATCH 09/13] fs/userfaultfd: use iov_iter for copy/zero Nadav Amit
2020-11-29  0:45 ` [RFC PATCH 10/13] fs/userfaultfd: add write_iter() interface Nadav Amit
2020-11-29  0:45 ` [RFC PATCH 11/13] fs/userfaultfd: complete write asynchronously Nadav Amit
2020-12-02  7:12   ` Nadav Amit
2020-11-29  0:45 ` [RFC PATCH 12/13] fs/userfaultfd: kmem-cache for wait-queue objects Nadav Amit
2020-11-30 19:51   ` Nadav Amit
2020-11-29  0:45 ` [RFC PATCH 13/13] selftests/vm/userfaultfd: iouring and polling tests Nadav Amit

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox