linux-kernel - Re: [PATCH v2 23/26] userfaultfd: wp: don't wake up when doing write protect

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Message-ID: <20190221183653.GV2813@redhat.com>
Date:   Thu, 21 Feb 2019 13:36:54 -0500
From:   Jerome Glisse <jglisse@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        David Hildenbrand <david@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        Maya Gokhale <gokhale2@...l.gov>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Martin Cracauer <cracauer@...s.org>, Shaohua Li <shli@...com>,
        Marty McFadden <mcfadden8@...l.gov>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Denis Plotnikov <dplotnikov@...tuozzo.com>,
        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 v2 23/26] userfaultfd: wp: don't wake up when doing write
 protect

On Tue, Feb 12, 2019 at 10:56:29AM +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.
> 
> Signed-off-by: Peter Xu <peterx@...hat.com>

I am bit confuse here, see below.

> ---
>  fs/userfaultfd.c | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 81962d62520c..f1f61a0278c2 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;
>  
>  	if (READ_ONCE(ctx->mmap_changing))
>  		return -EAGAIN;
> @@ -1789,18 +1790,20 @@ 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;

I am confuse by the logic here. DONTWAKE means do not wake any waiting
thread right ? So if the patch header it seems to me the logic should
be:
    if (mode_wp && !mode_dontwake)
        return -EINVAL;

At very least this part does seems to mean the opposite of what the
commit message says.

>  
>  	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,
>  				  &ctx->mmap_changing);
>  	if (ret)
>  		return ret;
>  
> -	if (!(uffdio_wp.mode & UFFDIO_WRITEPROTECT_MODE_DONTWAKE)) {
> +	if (!mode_wp && !mode_dontwake) {

This part match the commit message :)

>  		range.start = uffdio_wp.range.start;
>  		range.len = uffdio_wp.range.len;
>  		wake_userfault(ctx, &range);