Re: [PATCH] fs/userfaultfd.c: simplify the calculation of new_flags

[Peter Xu]

On Fri, Oct 04, 2019 at 07:28:34PM -0400, Andrea Arcangeli wrote:
> On Sat, Oct 05, 2019 at 06:46:40AM +0800, Wei Yang wrote:
> > On Wed, Oct 02, 2019 at 08:45:05PM -0400, Andrea Arcangeli wrote:
> > >Hello,
> > >
> > >On Tue, Aug 06, 2019 at 01:38:59PM +0800, Wei Yang wrote:
> > >> Finally new_flags equals old vm_flags *OR* vm_flags.
> > >> 
> > >> It is not necessary to mask them first.
> > >> 
> > >> Signed-off-by: Wei Yang <richardw.yang@xxxxxxxxxxxxxxx>
> > >> ---
> > >>  fs/userfaultfd.c | 2 +-
> > >>  1 file changed, 1 insertion(+), 1 deletion(-)
> > >> 
> > >> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> > >> index ccbdbd62f0d8..653d8f7c453c 100644
> > >> --- a/fs/userfaultfd.c
> > >> +++ b/fs/userfaultfd.c
> > >> @@ -1457,7 +1457,7 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
> > >>  			start = vma->vm_start;
> > >>  		vma_end = min(end, vma->vm_end);
> > >>  
> > >> -		new_flags = (vma->vm_flags & ~vm_flags) | vm_flags;
> > >> +		new_flags = vma->vm_flags | vm_flags;
> > >>  		prev = vma_merge(mm, prev, start, vma_end, new_flags,
> > >>  				 vma->anon_vma, vma->vm_file, vma->vm_pgoff,
> > >>  				 vma_policy(vma),
> > >
> > >And then how do you clear the flags after the above?
> > >
> > >It must be possible to clear the flags (from
> > >UFFDIO_REGISTER_MODE_MISSING|UFFDIO_REGISTER_MODE_WP to only one set
> > >or invert).
> > >
> > >We have no WP support upstream yet, so maybe that's why it looks
> > >superfluous in practice, but in theory it isn't because it would then
> > >need to be reversed by Peter's (CC'ed) -wp patchset.
> > >
> > >The register code has already the right placeholder to support -wp and
> > >so it's better not to break them.
> > >
> > >I would recommend reviewing the uffd-wp support and working on testing
> > >the uffd-wp code instead of changing the above.
> > >
> > 
> > Sorry, I don't get your point. This change is valid to me even from arithmetic
> > point of view.
> > 
> >     vm_flags == VM_UFFD_MISSING | VM_UFFD_WP
> > 
> > The effect of current code is clear these two bits then add them. This equals
> > to just add these two bits.
> > 
> > I am not sure which part I lost.
> 
> The cleaned removed the "& ~" and that was enough to quickly tell the
> cleaned up version was wrong.
> 
> What I should have noticed right away as well is that the code was
> already wrong, sorry. That code doesn't require a noop code cleanup,
> it requires a fix and the "& ~" needs to stay.
> 
> This isn't going to make any difference upstream until the uffd-wp
> support is merged so it is enough to queue it in Peter's queue, or you
> can merge it independently.

IMHO it's good to have it as independent patch so at least it won't
confuse another reader of the master branch.  But just in case, I've
also queued it in my local tree of uffd-wp.

Thanks,

-- 
Peter Xu