[PATCH v4] mm: Add PM_HUGE_THP_MAPPING to /proc/pid/pagemap

Mina Almasry
2021-11-07T23:58:00Z

Add PM_HUGE_THP MAPPING to allow userspace to detect whether a given virt
address is currently mapped by a transparent huge page or not.

Example use case is a process requesting THPs from the kernel (via
a huge tmpfs mount for example), for a performance critical region of
memory. The userspace may want to query whether the kernel is actually
backing this memory by hugepages or not.

PM_HUGE_THP_MAPPING bit is set if the virt address is mapped at the PMD
level and the underlying page is a transparent huge page.

Tested manually by adding logging into transhuge-stress, and by
allocating THP and querying the PM_HUGE_THP_MAPPING flag at those
virtual addresses.

Signed-off-by: Mina Almasry <almasrymina@google.com>

Cc: David Hildenbrand <david@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: David Rientjes rientjes@google.com
Cc: Paul E. McKenney <paulmckrcu@fb.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Ivan Teterevkov <ivan.teterevkov@nutanix.com>
Cc: Florian Schmidt <florian.schmidt@nutanix.com>
Cc: linux-kernel@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-mm@kvack.org

---

Changes in v4:
- Removed unnecessary moving of flags variable declaration

Changes in v3:
- Renamed PM_THP to PM_HUGE_THP_MAPPING
- Fixed checks to set PM_HUGE_THP_MAPPING
- Added PM_HUGE_THP_MAPPING docs

---
Documentation/admin-guide/mm/pagemap.rst | 3 ++-
fs/proc/task_mmu.c | 3 +++
tools/testing/selftests/vm/transhuge-stress.c | 21 +++++++++++++++----
3 files changed, 22 insertions(+), 5 deletions(-)

diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst
index fdc19fbc10839..8a0f0064ff336 100644
--- a/Documentation/admin-guide/mm/pagemap.rst
+++ b/Documentation/admin-guide/mm/pagemap.rst
@@ -23,7 +23,8 @@ There are four components to pagemap:
* Bit 56 page exclusively mapped (since 4.2)
* Bit 57 pte is uffd-wp write-protected (since 5.13) (see
:ref:`Documentation/admin-guide/mm/userfaultfd.rst <userfaultfd>`)
- * Bits 57-60 zero
+ * Bit 58 page is a huge (PMD size) THP mapping
+ * Bits 59-60 zero
* Bit 61 page is file-page or shared-anon (since 3.5)
* Bit 62 page swapped
* Bit 63 page present
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index ad667dbc96f5c..6f1403f83b310 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -1302,6 +1302,7 @@ struct pagemapread {
#define PM_SOFT_DIRTY BIT_ULL(55)
#define PM_MMAP_EXCLUSIVE BIT_ULL(56)
#define PM_UFFD_WP BIT_ULL(57)
+#define PM_HUGE_THP_MAPPING BIT_ULL(58)
#define PM_FILE BIT_ULL(61)
#define PM_SWAP BIT_ULL(62)
#define PM_PRESENT BIT_ULL(63)
@@ -1456,6 +1457,8 @@ static int pagemap_pmd_range(pmd_t *pmdp, unsigned long addr, unsigned long end,

if (page && page_mapcount(page) == 1)
flags |= PM_MMAP_EXCLUSIVE;
+ if (page && is_transparent_hugepage(page))
+ flags |= PM_HUGE_THP_MAPPING;

for (; addr != end; addr += PAGE_SIZE) {
pagemap_entry_t pme = make_pme(frame, flags);
diff --git a/tools/testing/selftests/vm/transhuge-stress.c b/tools/testing/selftests/vm/transhuge-stress.c
index fd7f1b4a96f94..7dce18981fff5 100644
--- a/tools/testing/selftests/vm/transhuge-stress.c
+++ b/tools/testing/selftests/vm/transhuge-stress.c
@@ -16,6 +16,12 @@
#include <string.h>
#include <sys/mman.h>

+/*
+ * We can use /proc/pid/pagemap to detect whether the kernel was able to find
+ * hugepages or no. This can be very noisy, so is disabled by default.
+ */
+#define NO_DETECT_HUGEPAGES
+
#define PAGE_SHIFT 12
#define HPAGE_SHIFT 21

@@ -23,6 +29,7 @@
#define HPAGE_SIZE (1 << HPAGE_SHIFT)

#define PAGEMAP_PRESENT(ent) (((ent) & (1ull << 63)) != 0)
+#define PAGEMAP_THP(ent) (((ent) & (1ull << 58)) != 0)
#define PAGEMAP_PFN(ent) ((ent) & ((1ull << 55) - 1))

int pagemap_fd;
@@ -47,10 +54,16 @@ int64_t allocate_transhuge(void *ptr)
(uintptr_t)ptr >> (PAGE_SHIFT - 3)) != sizeof(ent))
err(2, "read pagemap");

- if (PAGEMAP_PRESENT(ent[0]) && PAGEMAP_PRESENT(ent[1]) &&
- PAGEMAP_PFN(ent[0]) + 1 == PAGEMAP_PFN(ent[1]) &&
- !(PAGEMAP_PFN(ent[0]) & ((1 << (HPAGE_SHIFT - PAGE_SHIFT)) - 1)))
- return PAGEMAP_PFN(ent[0]);
+ if (PAGEMAP_PRESENT(ent[0]) && PAGEMAP_PRESENT(ent[1])) {
+#ifndef NO_DETECT_HUGEPAGES
+ if (!PAGEMAP_THP(ent[0]))
+ fprintf(stderr, "WARNING: detected non THP page\n");
+#endif
+ if (PAGEMAP_PFN(ent[0]) + 1 == PAGEMAP_PFN(ent[1]) &&
+ !(PAGEMAP_PFN(ent[0]) &
+ ((1 << (HPAGE_SHIFT - PAGE_SHIFT)) - 1)))
+ return PAGEMAP_PFN(ent[0]);
+ }

return -1;
}
--
2.34.0.rc0.344.g81b53c2807-goog