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

[Peter Xu]

On Thu, Feb 21, 2019 at 01:36:54PM -0500, Jerome Glisse wrote:
> 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@xxxxxxxxxx>
> 
> 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))

[1]

> > +
> > +	mode_wp = uffdio_wp.mode & UFFDIO_WRITEPROTECT_MODE_WP;
> > +	mode_dontwake = uffdio_wp.mode & UFFDIO_WRITEPROTECT_MODE_DONTWAKE;
> > +
> > +	if (mode_wp && mode_dontwake)

[2]

> >  		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;

This should be the most common case when we want to write protect a
page (or a set of pages).  I'll explain more details below...

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

Let me paste the matrix to be clear on these flags:

  |------+-------------------------+------------------------------|
  |      | dontwake=0              | dontwake=1                   |
  |------+-------------------------+------------------------------|
  | wp=0 | (a) resolve pf, do wake | (b) resolve pf only, no wake |
  | wp=1 | (c) wp page range       | (d) invalid                  |
  |------+-------------------------+------------------------------|

Above check at [1] was checking against case (d) in the matrix.  It is
indeed an invalid condition because when we want to write protect a
page we should not try to wake up any thread, so the donewake
parameter is actually useless (we'll always do that).  And above [2]
is simply rewritting [1] with the new variables.

> 
> >  
> >  	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 :)

Here is what the patch really want to change: before this patch we'll
even call wake_userfault() below for case (c) while it doesn't really
make too much sense IMHO.  After this patch we'll only do the wakeup
for (a,b).

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

Thanks,

-- 
Peter Xu