On Sat, 2025-08-16 at 14:43 +0000, Wei Yang wrote:
Hi Wei,
On Sat, Aug 16, 2025 at 09:31:11AM +0530, Aboorva Devarajan wrote:
In ksm_functional_tests, test_child_ksm() returned negative values to indicate errors. However, when passed to exit(), these were interpreted as large unsigned values (e.g, -2 became 254), leading to incorrect handling in the parent process. As a result, some tests appeared to be skipped or silently failed.
This is because "the least significant 8 bits" is returned to parent, right?
Yes, that's right. As per the WEXITSTATUS(wstatus) manual:
WEXITSTATUS: returns the exit status of the child. This consists of the least significant 8 bits of the status argument that the child specified in a call to exit(3) or _exit(2) or as the argument for a return statement in main(). This macro should only be employed if WIFEXITED returned true.
Since only the least significant 8 bits are preserved, negative return values can appear as large unsigned codes, so using small positive exit codes ensures the parent interprets the error code correctly.
This patch changes test_child_ksm() to return positive error codes (1, 2, 3) and updates test_child_ksm_err() to interpret them correctly. Additionally, test_prctl_fork_exec() now uses exit(4) after a failed execv() to clearly signal exec failures. This ensures the parent accurately detects and reports child process failures.
Before patch:
- [RUN] test_unmerge
ok 1 Pages were unmerged ...
- [RUN] test_prctl_fork
- No pages got merged
- [RUN] test_prctl_fork_exec
ok 7 PR_SET_MEMORY_MERGE value is inherited ... Bail out! 1 out of 8 tests failed
- Planned tests != run tests (9 != 8)
- Totals: pass:7 fail:1 xfail:0 xpass:0 skip:0 error:0
After patch:
- [RUN] test_unmerge
ok 1 Pages were unmerged ...
- [RUN] test_prctl_fork
- No pages got merged
not ok 7 Merge in child failed
- [RUN] test_prctl_fork_exec
ok 8 PR_SET_MEMORY_MERGE value is inherited ... Bail out! 2 out of 9 tests failed
- Totals: pass:7 fail:2 xfail:0 xpass:0 skip:0 error:0
Fixes: 6c47de3be3a0 ("selftest/mm: ksm_functional_tests: extend test case for ksm fork/exec") Co-developed-by: Donet Tom donettom@linux.ibm.com Signed-off-by: Donet Tom donettom@linux.ibm.com Acked-by: David Hildenbrand david@redhat.com Signed-off-by: Aboorva Devarajan aboorvad@linux.ibm.com
If so:
Reviewed-by: Wei Yang richard.weiyang@gmail.com
Thanks, I am afraid to make the same mistake if you don't point out.
Thanks, Aboorva