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SubjectRe: [PATCH RFC 20/24] userfaultfd: wp: don't wake up when doing write protect
On Mon, Jan 21, 2019 at 03:57:18PM +0800, Peter Xu wrote:
> It does not make sense to try to wake up any waiting thread when we're
> write-protecting a memory region. Only wake up when resolving a write
> protected page fault.

Probably it would be better to make it default to wake up only when
requested explicitly?
Then we can simply disallow _DONTWAKE for uffd_wp and only use
UFFDIO_WRITEPROTECT_MODE_WP as possible mode.

> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
> fs/userfaultfd.c | 13 ++++++++-----
> 1 file changed, 8 insertions(+), 5 deletions(-)
>
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 455b87c0596f..e54ab6076e13 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -1771,6 +1771,7 @@ static int userfaultfd_writeprotect(struct userfaultfd_ctx *ctx,
> struct uffdio_writeprotect uffdio_wp;
> struct uffdio_writeprotect __user *user_uffdio_wp;
> struct userfaultfd_wake_range range;
> + bool mode_wp, mode_dontwake;
>
> user_uffdio_wp = (struct uffdio_writeprotect __user *) arg;
>
> @@ -1786,17 +1787,19 @@ static int userfaultfd_writeprotect(struct userfaultfd_ctx *ctx,
> if (uffdio_wp.mode & ~(UFFDIO_WRITEPROTECT_MODE_DONTWAKE |
> UFFDIO_WRITEPROTECT_MODE_WP))
> return -EINVAL;
> - if ((uffdio_wp.mode & UFFDIO_WRITEPROTECT_MODE_WP) &&
> - (uffdio_wp.mode & UFFDIO_WRITEPROTECT_MODE_DONTWAKE))
> +
> + mode_wp = uffdio_wp.mode & UFFDIO_WRITEPROTECT_MODE_WP;
> + mode_dontwake = uffdio_wp.mode & UFFDIO_WRITEPROTECT_MODE_DONTWAKE;
> +
> + if (mode_wp && mode_dontwake)
> return -EINVAL;
>
> ret = mwriteprotect_range(ctx->mm, uffdio_wp.range.start,
> - uffdio_wp.range.len, uffdio_wp.mode &
> - UFFDIO_WRITEPROTECT_MODE_WP);
> + uffdio_wp.range.len, mode_wp);
> if (ret)
> return ret;
>
> - if (!(uffdio_wp.mode & UFFDIO_WRITEPROTECT_MODE_DONTWAKE)) {
> + if (!mode_wp && !mode_dontwake) {
> range.start = uffdio_wp.range.start;
> range.len = uffdio_wp.range.len;
> wake_userfault(ctx, &range);
> --
> 2.17.1
>

--
Sincerely yours,
Mike.

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