[PATCH v1 8/9] util/mmap-alloc: support RAM_NORESERVE via MAP_NORESERVE

[David Hildenbrand <david@redhat.com>]

Let's support RAM_NORESERVE via MAP_NORESERVE. At least on Linux,
the flag has no effect on shared mappings - except for anonymous memory
and hugetlbfs.

Linux man page:
  "MAP_NORESERVE: Do not reserve swap space for this mapping. When swap
  space is reserved, one has the guarantee that it is possible to modify
  the mapping. When swap space is not reserved one might get SIGSEGV
  upon a write if no physical memory is available. See also the discussion
  of the file /proc/sys/vm/overcommit_memory in proc(5). In kernels before
  2.6, this flag had effect only for private writable mappings."

Note that the "guarantee" part is wrong with memory overcommit in Linux.

Also, in Linux hugetlbfs is treated differently - we configure reservation
of huge pages from the pool, not reservation of swap space (huge pages
cannot be swapped).

The rough behavior is [1]:
a) !Hugetlbfs:

  1) Without MAP_NORESERVE *or* with memory overcommit under Linux
     disabled ("/proc/sys/vm/overcommit_memory == 2"), the following
     accounting/reservation happens:
      For a file backed map
       SHARED or READ-only - 0 cost (the file is the map not swap)
       PRIVATE WRITABLE - size of mapping per instance

      For an anonymous or /dev/zero map
       SHARED   - size of mapping
       PRIVATE READ-only - 0 cost (but of little use)
       PRIVATE WRITABLE - size of mapping per instance

  2) With MAP_NORESERVE, no accounting/reservation happens.

b) Hugetlbfs:

  1) Without MAP_NORESERVE, huge pages are reserved.

  2) With MAP_NORESERVE, no huge pages are reserved.

Note: With "/proc/sys/vm/overcommit_memory == 0", we were already able
to configure it for !hugetlbfs globally; this toggle now allows
configuring it more fine-grained, not for the whole system.

The target use case is virtio-mem, which dynamically exposes memory
inside a large, sparse memory area to the VM.

[1] https://www.kernel.org/doc/Documentation/vm/overcommit-accounting

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 softmmu/physmem.c |  1 +
 util/mmap-alloc.c | 47 +++++++++++++++++++++++++++++++++++++++++++----
 2 files changed, 44 insertions(+), 4 deletions(-)

diff --git a/softmmu/physmem.c b/softmmu/physmem.c
index 9820d845c0..f45a85add1 100644
--- a/softmmu/physmem.c
+++ b/softmmu/physmem.c
@@ -2248,6 +2248,7 @@ void qemu_ram_remap(ram_addr_t addr, ram_addr_t length)
                 abort();
             } else {
                 flags = MAP_FIXED;
+                flags |= (block->flags & RAM_NORESERVE) ? MAP_NORESERVE : 0;
                 if (block->fd >= 0) {
                     flags |= (block->flags & RAM_SHARED ?
                               MAP_SHARED : MAP_PRIVATE);
diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c
index bb99843106..f77d9ca574 100644
--- a/util/mmap-alloc.c
+++ b/util/mmap-alloc.c
@@ -20,6 +20,7 @@
 #include "qemu/osdep.h"
 #include "qemu/mmap-alloc.h"
 #include "qemu/host-utils.h"
+#include "qemu/cutils.h"
 #include "qemu/error-report.h"
 
 #define HUGETLBFS_MAGIC       0x958458f6
@@ -120,7 +121,7 @@ static void *mmap_reserve(size_t size, int fd)
  * it accessible.
  */
 static void *mmap_activate(void *ptr, size_t size, int fd, bool readonly,
-                           bool shared, bool is_pmem)
+                           bool shared, bool is_pmem, bool noreserve)
 {
     const int prot = PROT_READ | (readonly ? 0 : PROT_WRITE);
     int map_sync_flags = 0;
@@ -129,6 +130,7 @@ static void *mmap_activate(void *ptr, size_t size, int fd, bool readonly,
 
     flags |= fd == -1 ? MAP_ANONYMOUS : 0;
     flags |= shared ? MAP_SHARED : MAP_PRIVATE;
+    flags |= noreserve ? MAP_NORESERVE : 0;
     if (shared && is_pmem) {
         map_sync_flags = MAP_SYNC | MAP_SHARED_VALIDATE;
     }
@@ -170,6 +172,43 @@ static inline size_t mmap_guard_pagesize(int fd)
 #endif
 }
 
+#define OVERCOMMIT_MEMORY_PATH "/proc/sys/vm/overcommit_memory"
+static bool map_noreserve_effective(int fd, bool shared)
+{
+#if defined(__linux__)
+    gchar *content = NULL;
+    const char *endptr;
+    unsigned int tmp;
+
+    /* hugetlbfs behaves differently */
+    if (qemu_fd_getpagesize(fd) != qemu_real_host_page_size) {
+        return true;
+    }
+
+    /* only private shared mappings are accounted (ignoring /dev/zero) */
+    if (fd != -1 && shared) {
+        return true;
+    }
+
+    if (g_file_get_contents(OVERCOMMIT_MEMORY_PATH, &content, NULL, NULL) &&
+        !qemu_strtoui(content, &endptr, 0, &tmp) &&
+        (!endptr || *endptr == '\n')) {
+        if (tmp == 2) {
+            error_report("Skipping reservation of swap space is not supported: "
+                         " \"" OVERCOMMIT_MEMORY_PATH "\" is \"2\"");
+            return false;
+        }
+        return true;
+    }
+    /* this interface has been around since Linux 2.6 */
+    error_report("Skipping reservation of swap space is not supported: "
+                 " Could not read: \"" OVERCOMMIT_MEMORY_PATH "\"");
+    return false;
+#else
+    return true;
+#endif
+}
+
 void *qemu_ram_mmap(int fd,
                     size_t size,
                     size_t align,
@@ -182,8 +221,7 @@ void *qemu_ram_mmap(int fd,
     size_t offset, total;
     void *ptr, *guardptr;
 
-    if (noreserve) {
-        error_report("Skipping reservation of swap space is not supported");
+    if (noreserve && !map_noreserve_effective(fd, shared)) {
         return MAP_FAILED;
     }
 
@@ -204,7 +242,8 @@ void *qemu_ram_mmap(int fd,
 
     offset = QEMU_ALIGN_UP((uintptr_t)guardptr, align) - (uintptr_t)guardptr;
 
-    ptr = mmap_activate(guardptr + offset, size, fd, readonly, shared, is_pmem);
+    ptr = mmap_activate(guardptr + offset, size, fd, readonly, shared, is_pmem,
+                        noreserve);
     if (ptr == MAP_FAILED) {
         munmap(guardptr, total);
         return MAP_FAILED;
-- 
2.29.2