linux-kernel - Re: [PATCH RFC 20/24] userfaultfd: wp: don't wake up when doing write protect

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Message-Id: <20190121111039.GB26461@rapoport-lnx>
Date:   Mon, 21 Jan 2019 13:10:39 +0200
From:   Mike Rapoport <rppt@...ux.ibm.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Hugh Dickins <hughd@...gle.com>,
        Maya Gokhale <gokhale2@...l.gov>,
        Jerome Glisse <jglisse@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Martin Cracauer <cracauer@...s.org>,
        Denis Plotnikov <dplotnikov@...tuozzo.com>,
        Shaohua Li <shli@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Pavel Emelyanov <xemul@...allels.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Marty McFadden <mcfadden8@...l.gov>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Mel Gorman <mgorman@...e.de>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>
Subject: Re: [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@...hat.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.