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Sun, 01 Feb 2026 16:55:37 -0800 (PST) From: Usama Arif To: ziy@nvidia.com, Andrew Morton , David Hildenbrand , lorenzo.stoakes@oracle.com, linux-mm@kvack.org Cc: hannes@cmpxchg.org, riel@surriel.com, shakeel.butt@linux.dev, kas@kernel.org, baohua@kernel.org, dev.jain@arm.com, baolin.wang@linux.alibaba.com, npache@redhat.com, Liam.Howlett@oracle.com, ryan.roberts@arm.com, vbabka@suse.cz, lance.yang@linux.dev, linux-kernel@vger.kernel.org, kernel-team@meta.com, Usama Arif Subject: [RFC 08/12] selftests/mm: add PUD THP fork COW test Date: Sun, 1 Feb 2026 16:50:25 -0800 Message-ID: <20260202005451.774496-9-usamaarif642@gmail.com> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20260202005451.774496-1-usamaarif642@gmail.com> References: <20260202005451.774496-1-usamaarif642@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" Add a test that allocates a PUD THP, forks a child process, and has the child write to the shared memory. This triggers the copy-on-write path which must split the PUD THP. The test verifies that both parent and child see correct data after the split. Signed-off-by: Usama Arif --- tools/testing/selftests/mm/pud_thp_test.c | 44 +++++++++++++++++++++++ 1 file changed, 44 insertions(+) diff --git a/tools/testing/selftests/mm/pud_thp_test.c b/tools/testing/self= tests/mm/pud_thp_test.c index 7a1f0b0f81468..27a509cd477d5 100644 --- a/tools/testing/selftests/mm/pud_thp_test.c +++ b/tools/testing/selftests/mm/pud_thp_test.c @@ -181,4 +181,48 @@ TEST_F(pud_thp, read_write_access) ASSERT_EQ(errors, 0); } =20 +/* + * Test: Fork and copy-on-write + * Verifies that COW correctly splits the PUD THP and isolates parent/child + */ +TEST_F(pud_thp, fork_cow) +{ + unsigned long *ptr =3D (unsigned long *)self->aligned; + unsigned char *bytes =3D (unsigned char *)self->aligned; + pid_t pid; + int status; + unsigned long split_after; + + /* Initialize memory with known pattern */ + memset(self->aligned, 0xCC, PUD_SIZE); + + pid =3D fork(); + ASSERT_GE(pid, 0); + + if (pid =3D=3D 0) { + /* Child: write to trigger COW */ + ptr[0] =3D 0x12345678UL; + + /* Verify write succeeded and rest of memory unchanged */ + if (ptr[0] !=3D 0x12345678UL) + _exit(1); + if (bytes[PAGE_SIZE] !=3D 0xCC) + _exit(2); + + _exit(0); + } + + /* Parent: wait for child */ + waitpid(pid, &status, 0); + ASSERT_TRUE(WIFEXITED(status)); + ASSERT_EQ(WEXITSTATUS(status), 0); + + /* Verify parent memory unchanged (COW should have given child a copy) */ + ASSERT_EQ(bytes[0], 0xCC); + + split_after =3D read_vmstat("thp_split_pud"); + TH_LOG("Fork COW completed (thp_split_pud: %lu -> %lu)", + self->split_before, split_after); +} + TEST_HARNESS_MAIN --=20 2.47.3