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

[Wei Yang]

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.
>

ok, I get your point.

>Thanks,
>Andrea
>
>>From a0f17bef184c6bb9b99294f202eefb50b6eb43cd Mon Sep 17 00:00:00 2001
>From: Andrea Arcangeli <aarcange@xxxxxxxxxx>
>Date: Fri, 4 Oct 2019 19:09:59 -0400
>Subject: [PATCH 1/1] uffd: wp: clear VM_UFFD_MISSING or VM_UFFD_WP during
> userfaultfd_register()
>
>If the registration is repeated without VM_UFFD_MISSING or VM_UFFD_WP
>they need to be cleared. Currently setting UFFDIO_REGISTER_MODE_WP
>returns -EINVAL, so this patch is a noop until the
>UFFDIO_REGISTER_MODE_WP support is applied.
>
>Reported-by: Wei Yang <richardw.yang@xxxxxxxxxxxxxxx>
>Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>

Reviewed-by: Wei Yang <richardw.yang@xxxxxxxxxxxxxxx>

>---
> fs/userfaultfd.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
>diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
>index fe6d804a38dc..97596bb65dd5 100644
>--- a/fs/userfaultfd.c
>+++ b/fs/userfaultfd.c
>@@ -1458,7 +1458,8 @@ 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_UFFD_MISSING|VM_UFFD_WP)) | vm_flags;
> 		prev = vma_merge(mm, prev, start, vma_end, new_flags,
> 				 vma->anon_vma, vma->vm_file, vma->vm_pgoff,
> 				 vma_policy(vma),

-- 
Wei Yang
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