Re: [PATCH 0/2] mm/khugepaged: fix sub-PMD MADV_COLLAPSE range handling

[Lance Yang <lance.yang@linux.dev>]

Hi,

scripts/get_maintainer.pl is your friend :)
Please use it to Cc the relevant maintainers and reviewers next time.

Cheers, Lance

On Thu, May 07, 2026 at 03:05:56PM +0800, Chen Wandun wrote:
>madvise_collapse() computes a THP-aligned window from the caller's range:
>
>  hstart = (start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK  /* round up  */
>  hend   =  end   &  HPAGE_PMD_MASK                    /* round down */
>
>When the caller's range is smaller than one PMD (2 MiB) and/or not
>PMD-aligned, hstart can end up greater than hend.  In that case the
>collapsing loop is correctly skipped, but the return value was computed
>as ((hend - hstart) >> HPAGE_PMD_SHIFT): with hstart > hend the
>subtraction wraps unsigned, producing a huge value, the comparison
>"thps != 0" fires, and -EINVAL is returned instead of 0.
>
>A concrete example:
>
>  /* both cover less than one THP; both should return 0 */
>  madvise(aligned, PAGE_SIZE, MADV_COLLAPSE);             /* OK, returns 0 */
>  madvise(aligned + PAGE_SIZE, PAGE_SIZE, MADV_COLLAPSE); /* returns -EINVAL */
>
>The fix moves the hstart/hend calculation before kmalloc_obj() and
>returns 0 early when hstart >= hend.  This also avoids the kmalloc,
>mmgrab(), and lru_add_drain_all() calls for ranges that trivially
>contain no PMD window.  The same effect could be achieved by only
>guarding the final return expression, but early-return keeps the
>no-op path free of the allocator and drain overhead.
>
>Patch 1 fixes the kernel bug.
>Patch 2 adds a selftest with two cases covering the hstart == hend
>(aligned, was already correct) and hstart > hend (unaligned, was
>broken) scenarios.
>
>Chen Wandun (2):
>  mm/khugepaged: fix spurious -EINVAL from sub-PMD MADV_COLLAPSE range
>  selftests/mm: add MADV_COLLAPSE sub-PMD range tests
>
> mm/khugepaged.c                               |   9 +-
> tools/testing/selftests/mm/.gitignore         |   1 +
> tools/testing/selftests/mm/Makefile           |   2 +
> .../selftests/mm/ksft_madv_collapse.sh        |   4 +
> .../selftests/mm/madv_collapse_range.c        | 141 ++++++++++++++++++
> tools/testing/selftests/mm/run_vmtests.sh     |   5 +
> 6 files changed, 159 insertions(+), 3 deletions(-)
> create mode 100755 tools/testing/selftests/mm/ksft_madv_collapse.sh
> create mode 100644 tools/testing/selftests/mm/madv_collapse_range.c
>
>-- 
>2.43.0
>
>