| Message ID | 20211110221120.3833294-1-almasrymina@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [v5] mm: Add PM_HUGE_THP_MAPPING to /proc/pid/pagemap | expand |
On Wed, 10 Nov 2021 14:11:20 -0800 Mina Almasry <almasrymina@google.com> wrote: > 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. > > 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_HUGE_THP_MAPPING flag at those > virtual addresses. > > --- 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 > + > > ... > > +#ifndef NO_DETECT_HUGEPAGES > + if (!PAGEMAP_THP(ent[0])) > + fprintf(stderr, "WARNING: detected non THP page\n"); > +#endif This looks like a developer thing. Is there any point in leaving it in the mainline code?
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; }
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. 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_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 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 +++ tools/testing/selftests/vm/transhuge-stress.c | 21 +++++++++++++++---- 3 files changed, 22 insertions(+), 5 deletions(-) -- 2.34.0.rc0.344.g81b53c2807-goog