[PATCH v3] mm: Add PM_HUGE_THP_MAPPING to /proc/pid/pagemap

[Mina Almasry]

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@xxxxxxxxxx>

Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: David Rientjes rientjes@xxxxxxxxxx
Cc: Paul E. McKenney <paulmckrcu@xxxxxx>
Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Cc: Ivan Teterevkov <ivan.teterevkov@xxxxxxxxxxx>
Cc: Florian Schmidt <florian.schmidt@xxxxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
Cc: linux-mm@xxxxxxxxx

---
 Documentation/admin-guide/mm/pagemap.rst      |  3 ++-
 fs/proc/task_mmu.c                            |  6 +++++-
 tools/testing/selftests/vm/transhuge-stress.c | 21 +++++++++++++++----
 3 files changed, 24 insertions(+), 6 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..e10b59064c0b9 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)
@@ -1409,12 +1410,13 @@ static int pagemap_pmd_range(pmd_t *pmdp, unsigned long addr, unsigned long end,
 	struct pagemapread *pm = walk->private;
 	spinlock_t *ptl;
 	pte_t *pte, *orig_pte;
+	u64 flags = 0;
 	int err = 0;

 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 	ptl = pmd_trans_huge_lock(pmdp, vma);
 	if (ptl) {
-		u64 flags = 0, frame = 0;
+		u64 frame = 0;
 		pmd_t pmd = *pmdp;
 		struct page *page = NULL;

@@ -1456,6 +1458,8 @@ 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