Re: [PATCH] kselftest: mm: fix potential failure for merged VMA in guard-regions

[Yeoreum Yun]

On Fri, Sep 11, 2026 at 01:59:31PM +0100, Lorenzo Stoakes (ARM) wrote:
> On Fri, Sep 11, 2026 at 01:35:34PM +0100, Yeoreum Yun wrote:
> > check_vmflag_guard() uses /proc/self/smaps to retrieve the VMA flags,
> > but this can fail if the mapping is merged with an adjacent VMA.
> >
> > To avoid this potential failure, first allocate a temporary region with
> > extra pages at both ends, unmap it, and then map the test region within
> > the temporary address range, leaving an unmapped page on each side to
> > prevent VMA merging.
> >
> > Signed-off-by: Yeoreum Yun <yeoreum.yun@xxxxxxx>
> > ---
> >  tools/testing/selftests/mm/guard-regions.c | 14 ++++++++++++--
> >  1 file changed, 12 insertions(+), 2 deletions(-)
> >
> > diff --git a/tools/testing/selftests/mm/guard-regions.c b/tools/testing/selftests/mm/guard-regions.c
> > index 5c8ec3ca75d7..a28a57d34e97 100644
> > --- a/tools/testing/selftests/mm/guard-regions.c
> > +++ b/tools/testing/selftests/mm/guard-regions.c
> > @@ -2257,8 +2257,18 @@ TEST_F(guard_regions, smaps)
> >  	char *ptr, *ptr2;
> >  	int i;
> >
> > -	/* Map a region. */
> > -	ptr = mmap_(self, variant, NULL, 10 * page_size, PROT_READ | PROT_WRITE, 0, 0);
> > +	/* Try to Map a region. */
> 
> Map -> map
> 
> > +	ptr = mmap_(self, variant, NULL, 12 * page_size, PROT_READ | PROT_WRITE, 0, 0);
> 
> Should be PROT_NONE otherwise it'll merge with the below.

It doesn't matter. since this memory is unmapped and then second map
at ptr + page_size.

IOW, though the first one is merged, it unammped and then 
the second is allocated at ptr + page_size, it wouldn't be merged:

  after unmap:
   [existing VMA][ 12 pages ][existing VMA]

  second:
   [exiting VMA] [hole (page)] [ 10 pages (for test)] [hole (page)] [existing VMA]

Am I missing something?

-- 
Sincerely,
Yeoreum Yun