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

[Yeoreum Yun <yeoreum.yun@arm.com>]

> On Fri, Sep 11, 2026 at 02:51:09PM +0100, Lorenzo Stoakes (ARM) wrote:
> > On Fri, Sep 11, 2026 at 02:34:37PM +0100, Yeoreum Yun wrote:
> > > 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@arm.com>
> > > > > ---
> > > > >  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?
> >
> > Yeah, unmapped (unfaulted) VMAs can be merged with mapped (faulted) VMAs.
> >
> > In general it's also better to be explicit by specifying distinct attributes
> > anyway to spell out clearly that the VMAs are intended to perform that task.
> 
> Oops, I missed that you immediately unmapped the VMA too :)
> 
> In that case it's fine but I'd still prefer it PROT_NONE to clearly single it
> out as a placeholder.
> 
> Also a comment above it like:
> 
> 	/* Map then unmap placeholder to avoid adjacent merges */

Might the ASSERT() made you miss the unmap :).

I'll post soon again.

Thanks!

-- 
Sincerely,
Yeoreum Yun