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[PATCH v5 08/14] util/mmap-alloc: Support RAM_NORESERVE via MAP_NORESERV
From: |
David Hildenbrand |
Subject: |
[PATCH v5 08/14] util/mmap-alloc: Support RAM_NORESERVE via MAP_NORESERVE under Linux |
Date: |
Tue, 13 Apr 2021 11:14:15 +0200 |
Let's support RAM_NORESERVE via MAP_NORESERVE on Linux. The flag has no
effect on most shared mappings - except for hugetlbfs and anonymous memory.
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
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
include/qemu/osdep.h | 3 ++
softmmu/physmem.c | 1 +
util/mmap-alloc.c | 69 ++++++++++++++++++++++++++++++++++++++++++--
3 files changed, 71 insertions(+), 2 deletions(-)
diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
index 877d9a5eff..d5c1860508 100644
--- a/include/qemu/osdep.h
+++ b/include/qemu/osdep.h
@@ -176,6 +176,9 @@ extern int daemon(int, int);
#ifndef MAP_FIXED_NOREPLACE
#define MAP_FIXED_NOREPLACE 0
#endif
+#ifndef MAP_NORESERVE
+#define MAP_NORESERVE 0
+#endif
#ifndef ENOMEDIUM
#define ENOMEDIUM ENODEV
#endif
diff --git a/softmmu/physmem.c b/softmmu/physmem.c
index 608ba75d18..0d4376b04e 100644
--- a/softmmu/physmem.c
+++ b/softmmu/physmem.c
@@ -2229,6 +2229,7 @@ void qemu_ram_remap(ram_addr_t addr, ram_addr_t length)
flags = MAP_FIXED;
flags |= block->flags & RAM_SHARED ?
MAP_SHARED : MAP_PRIVATE;
+ flags |= block->flags & RAM_NORESERVE ? MAP_NORESERVE : 0;
if (block->fd >= 0) {
area = mmap(vaddr, length, PROT_READ | PROT_WRITE,
flags, block->fd, offset);
diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c
index d0cf4aaee5..838e286ce5 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
@@ -83,6 +84,70 @@ size_t qemu_mempath_getpagesize(const char *mem_path)
return qemu_real_host_page_size;
}
+#define OVERCOMMIT_MEMORY_PATH "/proc/sys/vm/overcommit_memory"
+static bool map_noreserve_effective(int fd, uint32_t qemu_map_flags)
+{
+#if defined(__linux__)
+ const bool readonly = qemu_map_flags & QEMU_MAP_READONLY;
+ const bool shared = qemu_map_flags & QEMU_MAP_SHARED;
+ gchar *content = NULL;
+ const char *endptr;
+ unsigned int tmp;
+
+ /*
+ * hugeltb accounting is different than ordinary swap reservation:
+ * a) Hugetlb pages from the pool are reserved for both private and
+ * shared mappings. For shared mappings, all mappers have to specify
+ * MAP_NORESERVE.
+ * b) MAP_NORESERVE is not affected by /proc/sys/vm/overcommit_memory.
+ */
+ if (qemu_fd_getpagesize(fd) != qemu_real_host_page_size) {
+ return true;
+ }
+
+ /*
+ * Accountable mappings in the kernel that can be affected by MAP_NORESEVE
+ * are private writable mappings (see mm/mmap.c:accountable_mapping() in
+ * Linux). For all shared or readonly mappings, MAP_NORESERVE is always
+ * implicitly active -- no reservation; this includes shmem. The only
+ * exception is shared anonymous memory, it is accounted like private
+ * anonymous memory.
+ */
+ if (readonly || (shared && fd >= 0)) {
+ return true;
+ }
+
+ /*
+ * MAP_NORESERVE is globally ignored for applicable !hugetlb mappings when
+ * memory overcommit is set to "never". Sparse memory regions aren't really
+ * possible in this system configuration.
+ *
+ * Bail out now instead of silently committing way more memory than
+ * currently desired by the user.
+ */
+ 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;
+#endif
+ /*
+ * E.g., FreeBSD used to define MAP_NORESERVE, never implemented it,
+ * and removed it a while ago.
+ */
+ error_report("Skipping reservation of swap space is not supported");
+ return false;
+}
+
/*
* Reserve a new memory region of the requested size to be used for mapping
* from the given fd (if any).
@@ -131,13 +196,13 @@ static void *mmap_activate(void *ptr, size_t size, int fd,
int flags = MAP_FIXED;
void *activated_ptr;
- if (noreserve) {
- error_report("Skipping reservation of swap space is not supported");
+ if (noreserve && !map_noreserve_effective(fd, qemu_map_flags)) {
return MAP_FAILED;
}
flags |= fd == -1 ? MAP_ANONYMOUS : 0;
flags |= shared ? MAP_SHARED : MAP_PRIVATE;
+ flags |= noreserve ? MAP_NORESERVE : 0;
if (shared && sync) {
map_sync_flags = MAP_SYNC | MAP_SHARED_VALIDATE;
}
--
2.30.2
- Re: [PATCH v5 04/14] softmmu/memory: Pass ram_flags to qemu_ram_alloc_from_fd(), (continued)
[PATCH v5 05/14] softmmu/memory: Pass ram_flags to memory_region_init_ram_shared_nomigrate(), David Hildenbrand, 2021/04/13
[PATCH v5 07/14] memory: Introduce RAM_NORESERVE and wire it up in qemu_ram_mmap(), David Hildenbrand, 2021/04/13
[PATCH v5 06/14] util/mmap-alloc: Pass flags instead of separate bools to qemu_ram_mmap(), David Hildenbrand, 2021/04/13
[PATCH v5 08/14] util/mmap-alloc: Support RAM_NORESERVE via MAP_NORESERVE under Linux,
David Hildenbrand <=
[PATCH v5 09/14] hostmem: Wire up RAM_NORESERVE via "reserve" property, David Hildenbrand, 2021/04/13
[PATCH v5 10/14] qmp: Clarify memory backend properties returned via query-memdev, David Hildenbrand, 2021/04/13
[PATCH v5 11/14] qmp: Include "share" property of memory backends, David Hildenbrand, 2021/04/13
[PATCH v5 13/14] qmp: Include "reserve" property of memory backends, David Hildenbrand, 2021/04/13
[PATCH v5 14/14] hmp: Print "reserve" property of memory backends with "info memdev", David Hildenbrand, 2021/04/13
[PATCH v5 12/14] hmp: Print "share" property of memory backends with "info memdev", David Hildenbrand, 2021/04/13