[Qemu-devel] [PATCH 00/10] RFC: userfault

[Andrea Arcangeli]

Hello everyone,

There's a large CC list for this RFC because this adds two new
syscalls (userfaultfd and remap_anon_pages) and
MADV_USERFAULT/MADV_NOUSERFAULT, so suggestions on changes to the API
or on a completely different API if somebody has better ideas are
welcome now.

The combination of these features are what I would propose to
implement postcopy live migration in qemu, and in general demand
paging of remote memory, hosted in different cloud nodes.

The MADV_USERFAULT feature should be generic enough that it can
provide the userfaults to the Android volatile range feature too, on
access of reclaimed volatile pages.

If the access could ever happen in kernel context through syscalls
(not not just from userland context), then userfaultfd has to be used
to make the userfault unnoticeable to the syscall (no error will be
returned). This latter feature is more advanced than what volatile
ranges alone could do with SIGBUS so far (but it's optional, if the
process doesn't call userfaultfd, the regular SIGBUS will fire, if the
fd is closed SIGBUS will also fire for any blocked userfault that was
waiting a userfaultfd_write ack).

userfaultfd is also a generic enough feature, that it allows KVM to
implement postcopy live migration without having to modify a single
line of KVM kernel code. Guest async page faults, FOLL_NOWAIT and all
other GUP features works just fine in combination with userfaults
(userfaults trigger async page faults in the guest scheduler so those
guest processes that aren't waiting for userfaults can keep running in
the guest vcpus).

remap_anon_pages is the syscall to use to resolve the userfaults (it's
not mandatory, vmsplice will likely still be used in the case of local
postcopy live migration just to upgrade the qemu binary, but
remap_anon_pages is faster and ideal for transferring memory across
the network, it's zerocopy and doesn't touch the vma: it only holds
the mmap_sem for reading).

The current behavior of remap_anon_pages is very strict to avoid any
chance of memory corruption going unnoticed. mremap is not strict like
that: if there's a synchronization bug it would drop the destination
range silently resulting in subtle memory corruption for
example. remap_anon_pages would return -EEXIST in that case. If there
are holes in the source range remap_anon_pages will return -ENOENT.

If remap_anon_pages is used always with 2M naturally aligned
addresses, transparent hugepages will not be splitted. In there could
be 4k (or any size) holes in the 2M (or any size) source range,
remap_anon_pages should be used with the RAP_ALLOW_SRC_HOLES flag to
relax some of its strict checks (-ENOENT won't be returned if
RAP_ALLOW_SRC_HOLES is set, remap_anon_pages then will just behave as
a noop on any hole in the source range). This flag is generally useful
when implementing userfaults with THP granularity, but it shouldn't be
set if doing the userfaults with PAGE_SIZE granularity if the
developer wants to benefit from the strict -ENOENT behavior.

The remap_anon_pages syscall API is not vectored, as I expect it to be
used mainly for demand paging (where there can be just one faulting
range per userfault) or for large ranges (with the THP model as an
alternative to zapping re-dirtied pages with MADV_DONTNEED with 4k
granularity before starting the guest in the destination node) where
vectoring isn't going to provide much performance advantages (thanks
to the THP coarser granularity).

On the rmap side remap_anon_pages doesn't add much complexity: there's
no need of nonlinear anon vmas to support it because I added the
constraint that it will fail if the mapcount is more than 1. So in
general the source range of remap_anon_pages should be marked
MADV_DONTFORK to prevent any risk of failure if the process ever
forks (like qemu can in some case).

One part that hasn't been tested is the poll() syscall on the
userfaultfd because the postcopy migration thread currently is more
efficient waiting on blocking read()s (I'll write some code to test
poll() too). I also appended below a patch to trinity to exercise
remap_anon_pages and userfaultfd and it completes trinity
successfully.

The code can be found here:

git clone --reference linux 
git://git.kernel.org/pub/scm/linux/kernel/git/andrea/aa.git -b userfault 

The branch is rebased so you can get updates for example with:

git fetch && git checkout -f origin/userfault

Comments welcome, thanks!
Andrea

>From cbe940e13b4cead41e0f862b3abfa3814f235ec3 Mon Sep 17 00:00:00 2001
From: Andrea Arcangeli <address@hidden>
Date: Wed, 2 Jul 2014 18:32:35 +0200
Subject: [PATCH] add remap_anon_pages and userfaultfd

Signed-off-by: Andrea Arcangeli <address@hidden>
---
 include/syscalls-x86_64.h   |   2 +
 syscalls/remap_anon_pages.c | 100 ++++++++++++++++++++++++++++++++++++++++++++
 syscalls/syscalls.h         |   2 +
 syscalls/userfaultfd.c      |  12 ++++++
 4 files changed, 116 insertions(+)
 create mode 100644 syscalls/remap_anon_pages.c
 create mode 100644 syscalls/userfaultfd.c

diff --git a/include/syscalls-x86_64.h b/include/syscalls-x86_64.h
index e09df43..a5b3a88 100644
--- a/include/syscalls-x86_64.h
+++ b/include/syscalls-x86_64.h
@@ -324,4 +324,6 @@ struct syscalltable syscalls_x86_64[] = {
        { .entry = &syscall_sched_setattr },
        { .entry = &syscall_sched_getattr },
        { .entry = &syscall_renameat2 },
+       { .entry = &syscall_remap_anon_pages },
+       { .entry = &syscall_userfaultfd },
 };
diff --git a/syscalls/remap_anon_pages.c b/syscalls/remap_anon_pages.c
new file mode 100644
index 0000000..b1e9d3c
--- /dev/null
+++ b/syscalls/remap_anon_pages.c
@@ -0,0 +1,100 @@
+/*
+ * SYSCALL_DEFINE3(remap_anon_pages,
+               unsigned long, dst_start, unsigned long, src_start,
+               unsigned long, len)
+ */
+#include <stdlib.h>
+#include <asm/mman.h>
+#include <assert.h>
+#include "arch.h"
+#include "maps.h"
+#include "random.h"
+#include "sanitise.h"
+#include "shm.h"
+#include "syscall.h"
+#include "tables.h"
+#include "trinity.h"
+#include "utils.h"
+
+static const unsigned long alignments[] = {
+       1 * MB, 2 * MB, 4 * MB, 8 * MB,
+       10 * MB, 100 * MB,
+};
+
+static unsigned char *g_src, *g_dst;
+static unsigned long g_size;
+static int g_check;
+
+#define RAP_ALLOW_SRC_HOLES (1UL<<0)
+
+static void sanitise_remap_anon_pages(struct syscallrecord *rec)
+{
+       unsigned long size = alignments[rand() % ARRAY_SIZE(alignments)];
+       unsigned long max_rand;
+       if (rand_bool()) {
+               g_src = mmap(NULL, size, PROT_READ|PROT_WRITE,
+                            MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
+       } else
+               g_src = MAP_FAILED;
+       if (rand_bool()) {
+               g_dst = mmap(NULL, size, PROT_READ|PROT_WRITE,
+                            MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
+       } else
+               g_dst = MAP_FAILED;
+       g_size = size;
+       g_check = 1;
+
+       rec->a1 = (unsigned long) g_dst;
+       rec->a2 = (unsigned long) g_src;
+       rec->a3 = g_size;
+       rec->a4 = 0;
+
+       if (rand_bool())
+               max_rand = -1UL;
+       else
+               max_rand = g_size << 1;
+       if (rand_bool()) {
+               rec->a3 += (rand() % max_rand) - g_size;
+               g_check = 0;
+       }
+       if (rand_bool()) {
+               rec->a1 += (rand() % max_rand) - g_size;
+               g_check = 0;
+       }
+       if (rand_bool()) {
+               rec->a2 += (rand() % max_rand) - g_size;
+               g_check = 0;
+       }
+       if (rand_bool()) {
+               if (rand_bool()) {
+                       rec->a4 = rand();
+               } else
+                       rec->a4 = RAP_ALLOW_SRC_HOLES;
+       }
+       if (g_src != MAP_FAILED)
+               memset(g_src, 0xaa, size);
+}
+
+static void post_remap_anon_pages(struct syscallrecord *rec)
+{
+       if (g_check && !rec->retval) {
+               unsigned long size = g_size;
+               unsigned char *dst = g_dst;
+               while (size--)
+                       assert(dst[size] == 0xaaU);
+       }
+       munmap(g_src, g_size);
+       munmap(g_dst, g_size);
+}
+
+struct syscallentry syscall_remap_anon_pages = {
+       .name = "remap_anon_pages",
+       .num_args = 4,
+       .arg1name = "dst_start",
+       .arg2name = "src_start",
+       .arg3name = "len",
+       .arg4name = "flags",
+       .group = GROUP_VM,
+       .sanitise = sanitise_remap_anon_pages,
+       .post = post_remap_anon_pages,
+};
diff --git a/syscalls/syscalls.h b/syscalls/syscalls.h
index 114500c..b8eaa63 100644
--- a/syscalls/syscalls.h
+++ b/syscalls/syscalls.h
@@ -370,3 +370,5 @@ extern struct syscallentry syscall_sched_setattr;
 extern struct syscallentry syscall_sched_getattr;
 extern struct syscallentry syscall_renameat2;
 extern struct syscallentry syscall_kern_features;
+extern struct syscallentry syscall_remap_anon_pages;
+extern struct syscallentry syscall_userfaultfd;
diff --git a/syscalls/userfaultfd.c b/syscalls/userfaultfd.c
new file mode 100644
index 0000000..769fe78
--- /dev/null
+++ b/syscalls/userfaultfd.c
@@ -0,0 +1,12 @@
+/*
+ * SYSCALL_DEFINE1(userfaultfd, int, flags)
+ */
+#include "sanitise.h"
+
+struct syscallentry syscall_userfaultfd = {
+       .name = "userfaultfd",
+       .num_args = 1,
+       .arg1name = "flags",
+       .arg1type = ARG_LEN,
+       .rettype = RET_FD,
+};


Andrea Arcangeli (10):
  mm: madvise MADV_USERFAULT: prepare vm_flags to allow more than 32bits
  mm: madvise MADV_USERFAULT
  mm: PT lock: export double_pt_lock/unlock
  mm: rmap preparation for remap_anon_pages
  mm: swp_entry_swapcount
  mm: sys_remap_anon_pages
  waitqueue: add nr wake parameter to __wake_up_locked_key
  userfaultfd: add new syscall to provide memory externalization
  userfaultfd: make userfaultfd_write non blocking
  userfaultfd: use VM_FAULT_RETRY in handle_userfault()

 arch/alpha/include/uapi/asm/mman.h     |   3 +
 arch/mips/include/uapi/asm/mman.h      |   3 +
 arch/parisc/include/uapi/asm/mman.h    |   3 +
 arch/x86/syscalls/syscall_32.tbl       |   2 +
 arch/x86/syscalls/syscall_64.tbl       |   2 +
 arch/xtensa/include/uapi/asm/mman.h    |   3 +
 fs/Makefile                            |   1 +
 fs/proc/task_mmu.c                     |   5 +-
 fs/userfaultfd.c                       | 593 +++++++++++++++++++++++++++++++++
 include/linux/huge_mm.h                |  11 +-
 include/linux/ksm.h                    |   4 +-
 include/linux/mm.h                     |   5 +
 include/linux/mm_types.h               |   2 +-
 include/linux/swap.h                   |   6 +
 include/linux/syscalls.h               |   5 +
 include/linux/userfaultfd.h            |  42 +++
 include/linux/wait.h                   |   5 +-
 include/uapi/asm-generic/mman-common.h |   3 +
 init/Kconfig                           |  10 +
 kernel/sched/wait.c                    |   7 +-
 kernel/sys_ni.c                        |   2 +
 mm/fremap.c                            | 506 ++++++++++++++++++++++++++++
 mm/huge_memory.c                       | 209 ++++++++++--
 mm/ksm.c                               |   2 +-
 mm/madvise.c                           |  19 +-
 mm/memory.c                            |  14 +
 mm/mremap.c                            |   2 +-
 mm/rmap.c                              |   9 +
 mm/swapfile.c                          |  13 +
 net/sunrpc/sched.c                     |   2 +-
 30 files changed, 1447 insertions(+), 46 deletions(-)
 create mode 100644 fs/userfaultfd.c
 create mode 100644 include/linux/userfaultfd.h