> 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 <[email protected]> > > > > > --- > > > > > 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
