>
>
> On 9/15/26 5:21 PM, Yeoreum Yun wrote:
> > There are intermittent failures in collapse_max_ptes_swap() and
> > collapse_max_ptes_shared() when using the khugepaged_context:
> >
> > // while running ./khugepaged -s 2
> >
> > # Run test: collapse_max_ptes_shared (khugepaged:anon)
> > # Allocate huge page... OK
> > # Share huge page over fork()... OK
> > # Trigger CoW on page 1023 of 2048... OK
> > # Maybe collapse with max_ptes_shared exceeded.... OK
> > # Trigger CoW on page 1024 of 2048... Fail
> > Bail out! Unexpected huge page
> > # Planned tests != run tests (26 != 23)
> > # Totals: pass:23 fail:0 xfail:0 xpass:0 skip:0 error:0
> >
> > # Run test: collapse_max_ptes_swap (khugepaged:anon)
> > # Swapout 257 of 2048 pages... OK
> > # Maybe collapse with max_ptes_swap exceeded.... OK
> > # Swapout 256 of 2048 pages... OK
> > Bail out! Unexpected huge page
> > # Planned tests != run tests (26 != 17)
> > # Totals: pass:17 fail:0 xfail:0 xpass:0 skip:0 error:0
> >
> > This happens because khugepaged may collapse the pages before
> > wait_for_scan()
> > is called, causing a sanity check that expects uncollapsed pages to fail.
> >
> > For example, in collapse_max_ptes_swap(), after faulting the pages back in
> > and paging out up to max_ptes_swap pages, khugepaged may collapse them again
> > before c->collapse() is called.
> >
> > To prevent this, change the khugepaged setting from ALWAYS to MADVICE for
> > the affected tests, and mark the VMA with MADV_NOHUGEPAGE after it has been
> > collapsed by wait_for_scan(). This prevents khugepaged from collapsing it
> > again before c->collapse() is called.
> >
> > This failure was observed on NVIDIA Spark with 16KB page.
> >
> > Signed-off-by: Yeoreum Yun <[email protected]>
> > ---
> > tools/testing/selftests/mm/khugepaged.c | 15 +++++++++++++++
> > 1 file changed, 15 insertions(+)
> >
> > diff --git a/tools/testing/selftests/mm/khugepaged.c
> > b/tools/testing/selftests/mm/khugepaged.c
> > index c32244b565658..83e9386bbc842 100644
> > --- a/tools/testing/selftests/mm/khugepaged.c
> > +++ b/tools/testing/selftests/mm/khugepaged.c
> > @@ -578,6 +578,8 @@ static bool wait_for_scan(const char *msg, char *p,
> > size_t len,
> > usleep(TICK);
> > }
> > + madvise(p, len, MADV_NOHUGEPAGE);
>
> This looks incorrect to me and would reintroduce the previous problem.
> Please see commit 7962e05a835f ("selftests: khugepaged: fix the shmem
> collapse failure").
But, after the thp_enabled changed the THP_MADVISE and then fault,
there is the chance to be collpased for anon.
I overlook the shmem case. but simple could we do like:
diff --git a/tools/testing/selftests/mm/khugepaged.c
b/tools/testing/selftests/mm/khugepaged.c
index c32244b565658..047524f001532 100644
--- a/tools/testing/selftests/mm/khugepaged.c
+++ b/tools/testing/selftests/mm/khugepaged.c
@@ -578,6 +578,9 @@ static bool wait_for_scan(const char *msg, char *p, size_t
len,
usleep(TICK);
}
+ if (!strncmp(ops->name, "anon", 4))
+ madvise(p, len, MADV_NOHUGEPAGE);
+
return timeout == -1;
}
>
> > +
> > return timeout == -1;
> > }
> > @@ -839,6 +841,7 @@ static void collapse_swapin_single_pte(struct
> > collapse_context *c, struct mem_op
> > static void collapse_max_ptes_swap(struct collapse_context *c, struct
> > mem_ops *ops)
> > {
> > + struct thp_settings settings = *thp_current_settings();
> > int max_ptes_swap = thp_read_num("khugepaged/max_ptes_swap");
> > void *p;
> > @@ -860,6 +863,9 @@ static void collapse_max_ptes_swap(struct
> > collapse_context *c, struct mem_ops *o
> > validate_memory(p, 0, hpage_pmd_size);
> > if (c->enforce_pte_scan_limits) {
> > + settings.hugepages[collapse_order].enabled = THP_MADVISE;
> > + thp_push_settings(&settings);
>
> I'm not sure why the collapse_order setting needs to be changed here. In
> your test case, you did not use the '-c' parameter to specify the collapse
> order.
What a stupid of me and I post wrong one... thie should be thp_enabled...
Apologise :(
>
> > +
> > ops->fault(p, 0, hpage_pmd_size);
> > ksft_print_msg("Swapout %d of %d pages...", max_ptes_swap,
> > hpage_pmd_nr);
> > @@ -869,12 +875,15 @@ static void collapse_max_ptes_swap(struct
> > collapse_context *c, struct mem_ops *o
> > success("OK");
> > } else {
> > fail("Fail");
> > + thp_pop_settings();
> > goto out;
> > }
> > c->collapse("Collapse with max_ptes_swap pages swapped out", p,
> > 1, ops, true);
> > validate_memory(p, 0, hpage_pmd_size);
> > +
> > + thp_pop_settings();
> > }
> > out:
> > ops->cleanup_area(p, hpage_pmd_size);
> > @@ -1075,6 +1084,7 @@ static void collapse_fork_compound(struct
> > collapse_context *c, struct mem_ops *o
> > static void collapse_max_ptes_shared(struct collapse_context *c, struct
> > mem_ops *ops)
> > {
> > + struct thp_settings settings = *thp_current_settings();
> > int max_ptes_shared = thp_read_num("khugepaged/max_ptes_shared");
> > int wstatus;
> > void *p;
> > @@ -1100,6 +1110,9 @@ static void collapse_max_ptes_shared(struct
> > collapse_context *c, struct mem_ops
> > 1, ops, !c->enforce_pte_scan_limits);
> > if (c->enforce_pte_scan_limits) {
> > + settings.hugepages[collapse_order].enabled =
> > THP_MADVISE;
> > + thp_push_settings(&settings);
>
> Ditto.
Sorry this should be thp_enabled...
>
> > +
> > ksft_print_msg("Trigger CoW on page %d of %d...",
> > hpage_pmd_nr - max_ptes_shared, hpage_pmd_nr);
> > ops->fault(p, 0, (hpage_pmd_nr - max_ptes_shared) *
> > @@ -1111,6 +1124,8 @@ static void collapse_max_ptes_shared(struct
> > collapse_context *c, struct mem_ops
> > c->collapse("Collapse with max_ptes_shared PTEs shared",
> > p, 1, ops, true);
> > +
> > + thp_pop_settings();
> > }
> > validate_memory(p, 0, hpage_pmd_size);
> >
>
Thanks!
--
Sincerely,
Yeoreum Yun
