Re: [PATCH v2 1/1] selftests/proc: use RSS in /proc/pid/smaps_rollup tearing tests

[Suren Baghdasaryan]

On Mon, Nov 2, 2026 at 5:08 AM Pedro Falcato <pfalcato@xxxxxxx> wrote:
>
> On Sun, Nov 01, 2026 at 11:01:00AM -0800, Andrew Morton wrote:
> > On Sun,  1 Nov 2026 07:45:03 -0800 Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
> >
> > > Pss metrics can be affected by map count even if VMA tree stays unchanged,
> > > therefore tests checking VMA tree integrity can't use them. Use Rss instead
> > > and pre-populate the VMAs so that Rss changes if a VMA gets skipped or
> > > double-counted.
> > >
> > > To ensure all test VMAs are populated with anonymous pages, remain
> > > mergeable across VMA operations, and do not have their Rss changed by
> > > memory reclaim:
> > > - Toggle PROT_EXEC instead of PROT_WRITE between adjacent test VMAs so
> > >   all test VMAs remain writable and MAP_POPULATE faults in anonymous pages
> > >   (rather than the shared zero page, which is not accounted in Rss).
> > > - Use mprotect() instead of mmap(..., MAP_FIXED) in split_vma() and
> > >   merge_vma() so the first page of the split VMA stays resident.
> > >   mprotect(..., PROT_NONE) splits the VMA without unmapping the first page
> > >   (pte_present() remains true for PAGE_NONE PTEs), and
> > >   mprotect(..., self->mod_info->prot) merges it back. Both parts of the
> > >   split VMA retain their non-zero Rss.
> > > - Lock all present pages in the child with mlockall(MCL_CURRENT |
> > >   MCL_FUTURE | MCL_ONFAULT) so memory pressure cannot reclaim pages and
> > >   alter Rss during the test.
> > > - In remap_vma(), unlock next_addr before mremap(..., MREMAP_DONTUNMAP)
> > >   so next_addr is not split, use mmap(..., MAP_FIXED) in patch_vma() to
> > >   merge the middle VMA back into addr, and exclude smaps_rollup tests
> > >   from test_maps_tearing_from_remap since remapping moves and unmaps the
> > >   page from next_addr.
> >
> > Thanks.  This is proving tricky.
> >
> > My evolving reviewbot still isn't happy:
> >
> > : This fixes the v1 RSS issues, but two things remain.
> > :
> > : Alternating PROT_EXEC breaks with READ_IMPLIES_EXEC: both mappings
>
> This is nonsense btw. Not sure we'll ever support random personality()
> flags being set on selftests.

Yeah, handling different personality() cases on a selftest sounds like
a stretch. Is this case realistic and do we need to handle it?

>
> --
> Pedro