* [PATCH v4 2/2] mm: selftests: Adjust the MADV_COLLAPSE selftests for uffd-minor
2026-10-03 0:18 [PATCH v4 1/2] mm/khugepaged: Never install PMDs in uffd-minor-registered VMAs James Houghton
@ 2026-10-03 0:18 ` James Houghton
0 siblings, 0 replies; 2+ messages in thread
From: James Houghton @ 2026-10-03 0:18 UTC (permalink / raw)
To: Andrew Morton
Cc: David Hildenbrand, Lorenzo Stoakes, Zi Yan, Baolin Wang, liam,
Nico Pache, Ryan Roberts, Dev Jain, Barry Song, Lance Yang,
Usama Arif, Yang Shi, zokeefe, hughd, Kiryl Shutsemau,
jthoughton, linux-mm, linux-kernel
The behavior of MADV_COLLAPSE with uffd-minor VMAs has changed.
Previously, MADV_COLLAPSE would install PMDs in parts of a VMA where
PTEs may be missing, essentially suppressing userfaultfd minor faults.
Now, MADV_COLLAPSE returns -EINVAL for uffd-minor VMAs. Update the
selftest to demonstrate this new behavior.
The selftest, if left unmodified, will SKIP when it gets EINVAL back
from MADV_COLLAPSE.
---
tools/testing/selftests/mm/uffd-unit-tests.c | 69 ++++++++++++++++++--
1 file changed, 62 insertions(+), 7 deletions(-)
diff --git a/tools/testing/selftests/mm/uffd-unit-tests.c b/tools/testing/selftests/mm/uffd-unit-tests.c
index 580178630ede..ab195c2c44d3 100644
--- a/tools/testing/selftests/mm/uffd-unit-tests.c
+++ b/tools/testing/selftests/mm/uffd-unit-tests.c
@@ -518,19 +518,32 @@ static void uffd_wp_fork_pin_with_event_test(uffd_global_test_opts_t *gopts, uff
uffd_wp_fork_pin_test_common(gopts, args, true);
}
-static void check_memory_contents(uffd_global_test_opts_t *gopts, char *p)
+static int __check_memory_contents(unsigned long offset, unsigned long nr_pages,
+ uffd_global_test_opts_t *gopts, char *p)
{
unsigned long i, j;
uint8_t expected_byte;
- for (i = 0; i < gopts->nr_pages; ++i) {
+ if (nr_pages + offset < nr_pages)
+ err("overflow in memory check");
+ if (nr_pages + offset > gopts->nr_pages)
+ err("out of bounds memory check");
+
+ for (i = offset; i < offset + nr_pages; ++i) {
expected_byte = ~((uint8_t)(i % ((uint8_t)-1)));
for (j = 0; j < gopts->page_size; j++) {
uint8_t v = *(uint8_t *)(p + (i * gopts->page_size) + j);
if (v != expected_byte)
- err("unexpected page contents");
+ return 1;
}
}
+
+ return 0;
+}
+
+static int check_memory_contents(uffd_global_test_opts_t *gopts, char *p)
+{
+ return __check_memory_contents(0, gopts->nr_pages, gopts, p);
}
static void uffd_minor_test_common(uffd_global_test_opts_t *gopts, bool test_collapse, bool test_wp)
@@ -538,8 +551,8 @@ static void uffd_minor_test_common(uffd_global_test_opts_t *gopts, bool test_col
unsigned long p;
pthread_t uffd_mon;
char c = '\0';
- struct uffd_args args = { 0 };
- args.gopts = gopts;
+ struct uffd_args args = { .gopts = gopts };
+ bool bad_contents;
/*
* NOTE: MADV_COLLAPSE is not yet compatible with WP, so testing
@@ -564,20 +577,59 @@ static void uffd_minor_test_common(uffd_global_test_opts_t *gopts, bool test_col
if (pthread_create(&uffd_mon, NULL, uffd_poll_thread, &args))
err("uffd_poll_thread create");
+ if (test_collapse) {
+ /*
+ * Read just a single page and try collapsing. The collapse
+ * should either be rejected or be a no-op.
+ */
+ if (__check_memory_contents(0, 1, gopts, gopts->area_dst_alias))
+ err("unexpected memory contents before collapse");
+
+ /* MADV_COLLAPSE might return EINVAL for uffd-minor VMAs. */
+ if (madvise(gopts->area_dst_alias, gopts->nr_pages * gopts->page_size,
+ MADV_COLLAPSE) && errno != EINVAL)
+ err("MADV_COLLAPSE failed unexpectedly");
+ /*
+ * If the above collapse mapped pages that were not explicitly
+ * CONTINUE'd, the below __check_memory_contents() will not
+ * fault on some pages, resulting in incorrect contents.
+ * The page we already touched should not be unmapped, so a
+ * second userfault on it should not be generated.
+ */
+ }
+
/*
* Read each of the pages back using the UFFD-registered mapping. We
* expect that the first time we touch a page, it will result in a minor
* fault. uffd_poll_thread will resolve the fault by bit-flipping the
* page's contents, and then issuing a CONTINUE ioctl.
*/
- check_memory_contents(gopts, gopts->area_dst_alias);
+ bad_contents = !!check_memory_contents(gopts, gopts->area_dst_alias);
if (write(gopts->pipefd[1], &c, sizeof(c)) != sizeof(c))
err("pipe write");
if (pthread_join(uffd_mon, NULL))
err("join() failed");
+ if (bad_contents && test_collapse) {
+ uffd_test_fail("unexpected memory contents after collapse");
+ return;
+ }
+
+ if (bad_contents) {
+ uffd_test_fail("unexpected memory contents");
+ return;
+ }
+
if (test_collapse) {
+ /*
+ * MADV_COLLAPSE will fail unless userfaultfd-minor is
+ * unregistered.
+ */
+ if (uffd_unregister(gopts->uffd, gopts->area_dst_alias,
+ gopts->nr_pages * gopts->page_size))
+ err("uffd_unregister before MADV_COLLAPSE failed");
+
if (madvise(gopts->area_dst_alias, gopts->nr_pages * gopts->page_size,
MADV_COLLAPSE)) {
/* It's fine to fail for this one... */
@@ -593,7 +645,10 @@ static void uffd_minor_test_common(uffd_global_test_opts_t *gopts, bool test_col
* This won't cause uffd-fault - it purely just makes sure there
* was no corruption.
*/
- check_memory_contents(gopts, gopts->area_dst_alias);
+ if (check_memory_contents(gopts, gopts->area_dst_alias)) {
+ uffd_test_fail("unexpected memory contents");
+ return;
+ }
}
if (args.missing_faults != 0 || args.minor_faults != gopts->nr_pages)
--
2.56.0.rc1.315.gc6ed9934b7-goog
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