Re: [PATCH v2 00/12] selftests/mm: improve mremap_test

[Lorenzo Stoakes (ARM)]

On Mon, Oct 12, 2026 at 10:20:45AM +0530, Sarthak Sharma wrote:
> On 10/6/26 2:54 PM, Sarthak Sharma wrote:
> > This series fixes correctness issues in mremap_test, simplifies its
> > test infrastructure and strengthens validation of remapped mappings.
> >
> > Patches 1-3 remove unused command line handling, unnecessary cleanup
> > checks and redundant errno handling. Patch 4 converts the test to use
> > kselftest helpers and removes manual failure tracking.
> >
> > Patches 5-8 fix handling of unavailable userfaultfd support, unexpected
> > mremap() successes, data-corruption failures and cleanup after mmap()
> > failures. Patch 9 removes perf tests and timing infrastructure.
> >
> > Patches 10 and 11 replace threshold-based random-data validation with
> > sampled validation and a deterministic data pattern. Patch 12
> > strengthens multi VMA validation by also checking the mapping state
> > of holes after remapping.
>
> [...]
>
> > ---
> > Sarthak Sharma (12):
> >   selftests/mm: mremap_test: remove command line options
> >   selftests/mm: mremap_test: drop munmap() checks from cleanup paths
> >   selftests/mm: mremap_test: simplify errno handling
> >   selftests/mm: mremap_test: use kselftest helpers
> >   selftests/mm: mremap_test: skip test when userfaultfd is unavailable
> >   selftests/mm: mremap_test: fail unexpected mremap successes
> >   selftests/mm: mremap_test: fail on data corruption
> >   selftests/mm: mremap_test: handle cleanup after mmap failures
> >   selftests/mm: mremap_test: remove perf tests and timing
> >   selftests/mm: mremap_test: replace validation threshold with sampled
> >     checks
> >   selftests/mm: mremap_test: replace random data with deterministic
> >     pattern
> >   selftests/mm: mremap_test: strengthen multi VMA validation
> >
> >  tools/testing/selftests/mm/mremap_test.c | 701 ++++++++++-------------
> >  1 file changed, 291 insertions(+), 410 deletions(-)
> >
>
> I wanted to address the concern Sashiko has over this patch series [1].
> It says that mremap(..., MAP_FIXED) over a range containing multiple
> VMAs should unmap the whole range first and then map the VMAs.

I don't think you need to spend time worrying about sashiko stuff about
mremap() behaviour in a test series :)

>
> But this is not the case, as was discussed on the list earlier as well.
> We iterate over the VMAs in the range and only unmap and then map the
> region that the VMAs occupy. IOW, as quoted by Lorenzo [2], "the
> sematics are such that gaps are not propagated (i.e. it is as if you
> mremap()'d each individually)". So if we remap over an existing mapping,
> the mapping region on which a hole is remapped should not be affected at
> all. So this is a false positive.

I will be removing the gaps stuff in any case (as discussed elsewhere
too).

It's problematic, more so for the fact stuff can get mapped in gaps before
the mremap() operation.

>
> While here, requesting for some feedback on this series, if anyone has
> had a chance to look at it :).

Thanks for the series, this cycle has been very busy so I think this is one
for the next cycle (i.e. targeting 7.5).

So I'd recommend resending at 7.4-rc1?

>
> [1]
> https://sashiko.dev/#/patchset/20261006092511.145776-1-sarthak.sharma%40arm.com
>
> [2] https://lore.kernel.org/linux-mm/agH1ZSdX_7QF8P_I@lucifer/
>

--
Cheers, Lorenzo