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

[Suren Baghdasaryan]

On Mon, Nov 2, 2026 at 5:26 AM Pedro Falcato <pfalcato@xxxxxxx> wrote:
>
> On Sun, Nov 01, 2026 at 07:45:03AM -0800, Suren Baghdasaryan 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.
>
> FWIW, I was wondering if it would remain constant in face of compaction (page
> migration). It seems so, from looking at the code the migration entry should
> make it count the RSS. But is it always the case? I'm not super familiar with
> the migration logic.

I'll double-check, but it seems logical to me that page migration
should not affect RSS.

>
> --
> Pedro