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Subject[PATCH v6] mm: Add PM_THP_MAPPED to /proc/pid/pagemap
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Add PM_THP_MAPPED 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_THP_MAPPED bit is set if the virt address is mapped at the PMD
level and the underlying page is a transparent huge page.

A few options were considered:
1. Add /proc/pid/pageflags that exports the same info as
/proc/kpageflags. This is not appropriate because many kpageflags are
inappropriate to expose to userspace processes.
2. Simply get this info from the existing /proc/pid/smaps interface.
There are a couple of issues with that:
1. /proc/pid/smaps output is human readable and unfriendly to
programatically parse.
2. /proc/pid/smaps is slow. The cost of reading /proc/pid/smaps into
userspace buffers is about ~800us per call, and this doesn't
include parsing the output to get the information you need. The
cost of querying 1 virt address in /proc/pid/pagemaps however is
around 5-7us.

Tested manually by adding logging into transhuge-stress, and by
allocating THP and querying the PM_THP_MAPPED 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 v6:
- Renamed to PM_THP_MAPPED
- Removed changes to transhuge-stress

Changes in v5:
- Added justification for this interface in the commit message!

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 +++
2 files changed, 5 insertions(+), 1 deletion(-)

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..d784a97aa209a 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_THP_MAPPED 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_THP_MAPPED;

for (; addr != end; addr += PAGE_SIZE) {
pagemap_entry_t pme = make_pme(frame, flags);
--
2.34.0.rc2.393.gf8c9666880-goog
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