On Sat, Aug 15, 2026 at 02:58:47AM +0100, Kiryl Shutsemau wrote:
> From: "Kiryl Shutsemau (Meta)" <[email protected]>
>
> collapse_swapin_single_pte() and collapse_max_ptes_swap() swap a range out
> and then require smaps to report exactly the count they asked for. Two
> things keep that count from arriving.
>
> MADV_PAGEOUT is best effort, so the count often turns up a moment late.
>
> And wait_for_scan() leaves MADV_HUGEPAGE behind, so khugepaged is still
> working on the range. Collapsing a range with up to max_ptes_swap pages
> swapped out means reading them back in, so the daemon empties the swap as
> fast as the case fills it. On arm64 with 64K pages max_ptes_swap is 1024
> pages, which is 64M a step, and the case loses:
>
> # Swapout 1024 of 8192 pages... Fail
> not ok 10 collapse_max_ptes_swap
>
> Ask again for up to two seconds, with the range held out of the daemon's
> reach while asking. The collapse each case runs next puts MADV_HUGEPAGE
> back, so only the setup is affected.
>
> If the pages still will not go, skip. A machine with no swap, or swap too
> small, full, capped by a memcg or busy with writeback, is not the kernel
> under test refusing. An error from madvise() itself still ends the run.
>
> Assisted-by: Claude-Code:claude-opus-5
> Reviewed-by: Muhammad Usama Anjum <[email protected]>
> Tested-by: Muhammad Usama Anjum <[email protected]>
> Signed-off-by: Kiryl Shutsemau (Meta) <[email protected]>
> ---
> tools/testing/selftests/mm/khugepaged.c | 53 +++++++++++++++++++------
> 1 file changed, 41 insertions(+), 12 deletions(-)
>
> diff --git a/tools/testing/selftests/mm/khugepaged.c
> b/tools/testing/selftests/mm/khugepaged.c
> index ec5c36a19d92..7eb9db0005a0 100644
> --- a/tools/testing/selftests/mm/khugepaged.c
> +++ b/tools/testing/selftests/mm/khugepaged.c
> @@ -219,6 +219,41 @@ static bool check_swap(void *addr, unsigned long size)
> return swap;
> }
>
> +/*
> + * Page the range out and wait for the swap count to say so.
> + *
> + * Two things get in the way. MADV_PAGEOUT is best effort:
> + * shrink_folio_list() leaves a folio alone when it cannot reclaim it right
> + * away, and one still under writeback from an earlier pageout is the common
> + * case, so the count the caller asks for arrives a moment later. And a
> range
> + * an earlier collapse left MADV_HUGEPAGE is one khugepaged is still working
> + * on: collapsing a range with up to max_ptes_swap pages swapped out means
> + * reading those pages back in, so the daemon undoes the pageout as fast as
> it
> + * is asked for. Keep the range out of its reach; the collapse the caller
> runs
> + * next puts MADV_HUGEPAGE back.
> + *
> + * Failing to get the pages out is the machine's answer, not the kernel's --
> + * swap too small, swap full, a memcg cap, a folio still under writeback --
> so
> + * callers skip rather than fail. An error from madvise() is different, and
> + * ends the run here.
> + */
This is a schloppy comment again. Please trim.
Walls of text are not wanted anywhere.
> +static bool swapout_range(void *p, unsigned long size)
> +{
> + int i;
> +
> + if (madvise(p, size, MADV_NOHUGEPAGE))
> + ksft_exit_fail_perror("madvise(MADV_NOHUGEPAGE)");
> +
> + for (i = 0; i < 40; i++) {
> + if (madvise(p, size, MADV_PAGEOUT))
> + ksft_exit_fail_perror("madvise(MADV_PAGEOUT)");
> + if (check_swap(p, size))
> + return true;
> + usleep(50 * 1000);
> + }
> + return false;
> +}
> +
> static void *alloc_mapping(int nr)
> {
> void *p;
> @@ -827,12 +862,10 @@ static void collapse_swapin_single_pte(struct
> collapse_context *c, struct mem_op
> ops->fault(p, 0, hpage_pmd_size);
>
> ksft_print_msg("Swapout one page...");
> - if (madvise(p, page_size, MADV_PAGEOUT))
> - ksft_exit_fail_perror("madvise(MADV_PAGEOUT)");
> - if (check_swap(p, page_size)) {
> + if (swapout_range(p, page_size)) {
> success("OK");
> } else {
> - fail("Fail");
> + skip("Could not swap out");
> goto out;
> }
>
> @@ -853,12 +886,10 @@ static void collapse_max_ptes_swap(struct
> collapse_context *c, struct mem_ops *o
> ops->fault(p, 0, hpage_pmd_size);
>
> ksft_print_msg("Swapout %d of %d pages...", max_ptes_swap + 1,
> hpage_pmd_nr);
> - if (madvise(p, (max_ptes_swap + 1) * page_size, MADV_PAGEOUT))
> - ksft_exit_fail_perror("madvise(MADV_PAGEOUT)");
> - if (check_swap(p, (max_ptes_swap + 1) * page_size)) {
> + if (swapout_range(p, (max_ptes_swap + 1) * page_size)) {
> success("OK");
> } else {
> - fail("Fail");
> + skip("Could not swap out");
> goto out;
> }
>
> @@ -870,12 +901,10 @@ static void collapse_max_ptes_swap(struct
> collapse_context *c, struct mem_ops *o
> ops->fault(p, 0, hpage_pmd_size);
> ksft_print_msg("Swapout %d of %d pages...", max_ptes_swap,
> hpage_pmd_nr);
> - if (madvise(p, max_ptes_swap * page_size, MADV_PAGEOUT))
> - ksft_exit_fail_perror("madvise(MADV_PAGEOUT)");
> - if (check_swap(p, max_ptes_swap * page_size)) {
> + if (swapout_range(p, max_ptes_swap * page_size)) {
> success("OK");
> } else {
> - fail("Fail");
> + skip("Could not swap out");
> goto out;
> }
>
> --
> 2.54.0
>
--
Cheers, Lorenzo
