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[173.79.60.52]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-91b97b95847sm58498006d6.32.2026.10.14.10.58.15 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 14 Oct 2026 10:58:15 -0700 (PDT) From: "Gregory Price (Meta)" To: linux-kselftest@vger.kernel.org, linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, kernel-team@meta.com, akpm@linux-foundation.org, david@kernel.org, ljs@kernel.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, shuah@kernel.org Subject: [PATCH 8/8] selftests/mm: check that unauthorized MADV_PAGEOUT skips large file folios Date: Wed, 14 Oct 2026 13:57:56 -0400 Message-ID: <20261014175756.2035176-9-gourry@gourry.net> X-Mailer: git-send-email 2.56.0 In-Reply-To: <20261014175756.2035176-1-gourry@gourry.net> References: <20261014175756.2035176-1-gourry@gourry.net> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit A caller that neither owns nor may write a file may page out the COW pages of a private mapping of it, but not its file pages. When the advised range only partly covers a large file folio, the folio must be filtered out before it is split or reclaimed, on separate paths for PMD- and PTE-mapped folios. Check that partial MADV_PAGEOUT by such a caller leaves every page of a PMD-sized shmem folio resident, both when a PMD maps it and when a mapping of half of it maps it with PTEs. Signed-off-by: Gregory Price (Meta) --- tools/testing/selftests/mm/madvise.c | 121 +++++++++++++++++++++++++-- 1 file changed, 115 insertions(+), 6 deletions(-) diff --git a/tools/testing/selftests/mm/madvise.c b/tools/testing/selftests/mm/madvise.c index 74202fed2d04..d59c03ec8191 100644 --- a/tools/testing/selftests/mm/madvise.c +++ b/tools/testing/selftests/mm/madvise.c @@ -88,10 +88,11 @@ static char *map_aligned_pages(struct __test_metadata *_metadata, size_t size) } /* - * Collapse a PMD-aligned range into PMD-mapped anonymous THPs. MADV_COLLAPSE - * may fail transiently with EAGAIN. + * Collapse a PMD-aligned range into PMD-mapped THPs, and confirm the result + * with check_huge(). MADV_COLLAPSE may fail transiently with EAGAIN. */ -static bool collapse(char *mapping, size_t size) +static bool collapse(char *mapping, size_t size, + bool (*check_huge)(void *, size_t, int, uint64_t)) { int nr_hpages = size / pmd_size; int retry; @@ -100,8 +101,7 @@ static bool collapse(char *mapping, size_t size) if (madvise(mapping, size, MADV_COLLAPSE)) { if (errno != EAGAIN) return false; - } else if (check_huge_anon(mapping, size, nr_hpages, - pmd_size)) { + } else if (check_huge(mapping, size, nr_hpages, pmd_size)) { return true; } usleep(10000); @@ -118,7 +118,7 @@ static char *map_thp(struct __test_metadata *_metadata) if (!pmd_size) return NULL; mapping = map_aligned_pages(_metadata, pmd_size); - if (collapse(mapping, pmd_size)) + if (collapse(mapping, pmd_size, check_huge_anon)) return mapping; ASSERT_EQ(munmap(mapping, pmd_size), 0); return NULL; @@ -147,6 +147,33 @@ static void pte_map_thp(struct __test_metadata *_metadata, char *mapping, ASSERT_TRUE(flags & KPF_COMPOUND_HEAD); } +static char *map_aligned_file(struct __test_metadata *_metadata, int fd, + size_t size, int flags) +{ + char *mapping = reserve_aligned(_metadata, size); + + ASSERT_EQ(mmap(mapping, size, PROT_READ | PROT_WRITE, flags | MAP_FIXED, + fd, 0), mapping); + + return mapping; +} + +/* Create a PMD-sized shmem file filled with 1s. */ +static int create_shmem_file(struct __test_metadata *_metadata) +{ + char *shared; + int fd; + + fd = memfd_create("madvise-pageout", 0); + ASSERT_GE(fd, 0); + ASSERT_EQ(ftruncate(fd, pmd_size), 0); + shared = map_aligned_file(_metadata, fd, pmd_size, MAP_SHARED); + memset(shared, 1, pmd_size); + ASSERT_EQ(munmap(shared, pmd_size), 0); + + return fd; +} + static void assert_filled(struct __test_metadata *_metadata, const char *mapping, size_t size) { @@ -734,4 +761,86 @@ TEST_F(madvise, pageout_private_file_reclaims_only_cow) ASSERT_EQ(munmap(mapping, 2 * page_size), 0); } +/* + * An unauthorized caller must not page out a large file folio of a private + * mapping, even when the advised range only partly covers it. See + * pageout_stays_within_range about other unmapping. + */ +TEST_F(madvise, pageout_private_file_skips_large_file_folio) +{ + char *mapping; + size_t offset; + int fd; + + if (!pmd_size) + SKIP(return, "PMD-sized THPs are not supported"); + if (geteuid()) + SKIP(return, "requires root to change credentials"); + fd = create_shmem_file(_metadata); + mapping = map_aligned_file(_metadata, fd, pmd_size, MAP_PRIVATE); + if (!collapse(mapping, pmd_size, check_huge_file)) + SKIP(return, "could not allocate a PMD-sized shmem THP"); + ASSERT_EQ(fchmod(fd, 0400), 0); + if (!drop_privileges()) + SKIP(return, "could not drop privileges"); + + ASSERT_EQ(madvise(mapping, pmd_size / 2, MADV_PAGEOUT), 0); + for (offset = 0; offset < pmd_size; offset += page_size) + ASSERT_TRUE(page_is_present(self->pagemap_fd, + mapping + offset)); + assert_filled(_metadata, mapping, pmd_size); + + ASSERT_EQ(munmap(mapping, pmd_size), 0); + close(fd); +} + +/* + * An unauthorized caller must not page out a large file folio that a private + * mapping maps with PTEs, even when the advised range covers only part of it. + * See pageout_stays_within_range about other unmapping. + */ +TEST_F(madvise, pageout_private_file_skips_pte_mapped_file_folio) +{ + const size_t size = pmd_size / 2; + char *mapping, *shared; + unsigned long pfn; + uint64_t flags; + size_t offset; + int fd; + + if (!pmd_size) + SKIP(return, "PMD-sized THPs are not supported"); + if (geteuid()) + SKIP(return, "requires root to change credentials"); + if (self->kpageflags_fd < 0) + SKIP(return, "requires root to read page flags"); + fd = create_shmem_file(_metadata); + shared = map_aligned_file(_metadata, fd, pmd_size, MAP_SHARED); + if (!collapse(shared, pmd_size, check_huge_file)) + SKIP(return, "could not allocate a PMD-sized shmem THP"); + ASSERT_EQ(munmap(shared, pmd_size), 0); + + /* Mapping half of the folio leaves it mapped by PTEs. */ + mapping = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0); + ASSERT_NE(mapping, MAP_FAILED); + assert_filled(_metadata, mapping, size); + pfn = pagemap_get_pfn(self->pagemap_fd, mapping); + ASSERT_NE(pfn, -1UL); + ASSERT_EQ(pageflags_get(pfn, self->kpageflags_fd, &flags), 0); + if (!(flags & KPF_COMPOUND_HEAD)) + SKIP(return, "the shmem THP was split"); + ASSERT_EQ(fchmod(fd, 0400), 0); + if (!drop_privileges()) + SKIP(return, "could not drop privileges"); + + ASSERT_EQ(madvise(mapping, size / 2, MADV_PAGEOUT), 0); + for (offset = 0; offset < size; offset += page_size) + ASSERT_TRUE(page_is_present(self->pagemap_fd, + mapping + offset)); + assert_filled(_metadata, mapping, size); + + ASSERT_EQ(munmap(mapping, size), 0); + close(fd); +} + TEST_HARNESS_MAIN -- 2.56.0