On 12/18/25 14:18, Kevin Brodsky wrote:
On 18/12/2025 09:05, David Hildenbrand (Red Hat) wrote:
On 12/16/25 15:56, Ryan Roberts wrote:
On 16/12/2025 14:26, Kevin Brodsky wrote:
One of the pagemap_ioctl tests attempts to fault in pages by
memcpy()'ing them to an unused buffer. This probably worked
originally, but since commit 46036188ea1f ("selftests/mm: build with
-O2") the compiler is free to optimise away that unused buffer and
the memcpy() with it. As a result there might not be any resident
page in the mapping and the test may fail.
We don't need to copy all that memory anyway. Just fault in every
page by forcing the compiler to read the first byte.
Cc: Usama Anjum <[email protected]>
Signed-off-by: Kevin Brodsky <[email protected]>
---
tools/testing/selftests/mm/pagemap_ioctl.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)
diff --git a/tools/testing/selftests/mm/pagemap_ioctl.c
b/tools/testing/selftests/mm/pagemap_ioctl.c
index 2cb5441f29c7..67a7a3705604 100644
--- a/tools/testing/selftests/mm/pagemap_ioctl.c
+++ b/tools/testing/selftests/mm/pagemap_ioctl.c
@@ -1056,7 +1056,6 @@ int sanity_tests(void)
struct page_region *vec;
char *mem, *fmem;
struct stat sbuf;
- char *tmp_buf;
/* 1. wrong operation */
mem_size = 10 * page_size;
@@ -1167,8 +1166,9 @@ int sanity_tests(void)
if (fmem == MAP_FAILED)
ksft_exit_fail_msg("error nomem %d %s\n", errno,
strerror(errno));
- tmp_buf = malloc(sbuf.st_size);
- memcpy(tmp_buf, fmem, sbuf.st_size);
+ /* Fault in every page by reading the first byte */
+ for (i = 0; i < sbuf.st_size; i += page_size)
+ (void)*(volatile char *)(fmem + i);
We have FORCE_READ() in vm_util.h for this. Perhaps that would be
better?
Agreed, and if we have multiple patterns where we want to force_read a
bigger area, maybe we should provide a helper for that?
I've found just a couple of cases where FORCE_READ() is used for a
larger area (in hugetlb-madvise.c and split_huge_page_test.c). The step
size isn't the same in any of these cases though. We could have
something like fault_area(addr, size, step) but maybe the loops are
clear enough already?
Note that even for hugtlb we can read page-per-page, no need to
hugetlb-page-per-hugetlb-page. Not sure if the performance change would
make any real performance difference in this testing code.
--
Cheers
David
