Let's sense support and use it for preallocation. MADV_POPULATE_WRITE
does not require a SIGBUS handler, doesn't actually touch page content,
and avoids context switches; it is, therefore, faster and easier to handle
than our current approach.
This resolves the TODO in do_touch_pages().
Signed-off-by: David Hildenbrand <da...@redhat.com>
---
include/qemu/osdep.h | 7 ++++
util/oslib-posix.c | 84 +++++++++++++++++++++++++++++++++-----------
2 files changed, 71 insertions(+), 20 deletions(-)
diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
index 60718fc342..d1660d67fa 100644
--- a/include/qemu/osdep.h
+++ b/include/qemu/osdep.h
@@ -471,6 +471,11 @@ static inline void qemu_cleanup_generic_vfree(void *p)
#else
#define QEMU_MADV_REMOVE QEMU_MADV_DONTNEED
#endif
+#ifdef MADV_POPULATE_WRITE
+#define QEMU_MADV_POPULATE_WRITE MADV_POPULATE_WRITE
+#else
+#define QEMU_MADV_POPULATE_WRITE QEMU_MADV_INVALID
+#endif
#elif defined(CONFIG_POSIX_MADVISE)
@@ -484,6 +489,7 @@ static inline void qemu_cleanup_generic_vfree(void *p)
#define QEMU_MADV_HUGEPAGE QEMU_MADV_INVALID
#define QEMU_MADV_NOHUGEPAGE QEMU_MADV_INVALID
#define QEMU_MADV_REMOVE QEMU_MADV_DONTNEED
+#define QEMU_MADV_POPULATE_WRITE QEMU_MADV_INVALID
#else /* no-op */
@@ -497,6 +503,7 @@ static inline void qemu_cleanup_generic_vfree(void *p)
#define QEMU_MADV_HUGEPAGE QEMU_MADV_INVALID
#define QEMU_MADV_NOHUGEPAGE QEMU_MADV_INVALID
#define QEMU_MADV_REMOVE QEMU_MADV_INVALID
+#define QEMU_MADV_POPULATE_WRITE QEMU_MADV_INVALID
#endif
diff --git a/util/oslib-posix.c b/util/oslib-posix.c
index e8bdb02e1d..679796ac1f 100644
--- a/util/oslib-posix.c
+++ b/util/oslib-posix.c
@@ -484,10 +484,6 @@ static void *do_touch_pages(void *arg)
*
* 'volatile' to stop compiler optimizing this away
* to a no-op
- *
- * TODO: get a better solution from kernel so we
- * don't need to write at all so we don't cause
- * wear on the storage backing the region...
*/
*(volatile char *)addr = *addr;
addr += hpagesize;
@@ -497,6 +493,27 @@ static void *do_touch_pages(void *arg)
return NULL;
}
+static void *do_madv_populate_write_pages(void *arg)
+{
+ MemsetThread *memset_args = (MemsetThread *)arg;
+ const size_t size = memset_args->numpages * memset_args->hpagesize;
+ char * const addr = memset_args->addr;
+ int ret;
+
+ /* See do_touch_pages(). */
+ qemu_mutex_lock(&page_mutex);
+ while (!threads_created_flag) {
+ qemu_cond_wait(&page_cond, &page_mutex);
+ }
+ qemu_mutex_unlock(&page_mutex);
+
+ ret = qemu_madvise(addr, size, QEMU_MADV_POPULATE_WRITE);
+ if (ret) {
+ memset_thread_failed = true;
+ }
+ return NULL;
+}
+
static inline int get_memset_num_threads(int smp_cpus)
{
long host_procs = sysconf(_SC_NPROCESSORS_ONLN);
@@ -510,10 +527,11 @@ static inline int get_memset_num_threads(int smp_cpus)
}
static bool touch_all_pages(char *area, size_t hpagesize, size_t numpages,
- int smp_cpus)
+ int smp_cpus, bool use_madv_populate_write)
{
static gsize initialized = 0;
size_t numpages_per_thread, leftover;
+ void *(*touch_fn)(void *);
char *addr = area;
int i = 0;
@@ -523,6 +541,12 @@ static bool touch_all_pages(char *area, size_t hpagesize,
size_t numpages,
g_once_init_leave(&initialized, 1);
}
+ if (use_madv_populate_write) {
+ touch_fn = do_madv_populate_write_pages;
+ } else {
+ touch_fn = do_touch_pages;
+ }
+
memset_thread_failed = false;
threads_created_flag = false;
memset_num_threads = get_memset_num_threads(smp_cpus);
@@ -534,7 +558,7 @@ static bool touch_all_pages(char *area, size_t hpagesize,
size_t numpages,
memset_thread[i].numpages = numpages_per_thread + (i < leftover);
memset_thread[i].hpagesize = hpagesize;
qemu_thread_create(&memset_thread[i].pgthread, "touch_pages",
- do_touch_pages, &memset_thread[i],
+ touch_fn, &memset_thread[i],
QEMU_THREAD_JOINABLE);
addr += memset_thread[i].numpages * hpagesize;
}
@@ -553,6 +577,12 @@ static bool touch_all_pages(char *area, size_t hpagesize,
size_t numpages,
return memset_thread_failed;
}
+static bool madv_populate_write_possible(char *area, size_t pagesize)
+{
+ return !qemu_madvise(area, pagesize, QEMU_MADV_POPULATE_WRITE) ||
+ errno != EINVAL;
+}
+
void os_mem_prealloc(int fd, char *area, size_t memory, int smp_cpus,
Error **errp)
{
@@ -560,29 +590,43 @@ void os_mem_prealloc(int fd, char *area, size_t memory,
int smp_cpus,
struct sigaction act, oldact;
size_t hpagesize = qemu_fd_getpagesize(fd);
size_t numpages = DIV_ROUND_UP(memory, hpagesize);
+ bool use_madv_populate_write;
+
+ /*
+ * Sense on every invocation, as MADV_POPULATE_WRITE cannot be used for
+ * some special mappings, such as mapping /dev/mem.
+ */
+ if (madv_populate_write_possible(area, hpagesize)) {
+ use_madv_populate_write = true;
+ }
- memset(&act, 0, sizeof(act));
- act.sa_handler = &sigbus_handler;
- act.sa_flags = 0;
+ if (!use_madv_populate_write) {
+ memset(&act, 0, sizeof(act));
+ act.sa_handler = &sigbus_handler;
+ act.sa_flags = 0;
- ret = sigaction(SIGBUS, &act, &oldact);
- if (ret) {
- error_setg_errno(errp, errno,
- "os_mem_prealloc: failed to install signal handler");
- return;
+ ret = sigaction(SIGBUS, &act, &oldact);
+ if (ret) {
+ error_setg_errno(errp, errno,
+ "os_mem_prealloc: failed to install signal handler");
+ return;
+ }
}
/* touch pages simultaneously */
- if (touch_all_pages(area, hpagesize, numpages, smp_cpus)) {
+ if (touch_all_pages(area, hpagesize, numpages, smp_cpus,
+ use_madv_populate_write)) {
error_setg(errp, "os_mem_prealloc: Insufficient free host memory "
"pages available to allocate guest RAM");
}
- ret = sigaction(SIGBUS, &oldact, NULL);
- if (ret) {
- /* Terminate QEMU since it can't recover from error */
- perror("os_mem_prealloc: failed to reinstall signal handler");
- exit(1);
+ if (!use_madv_populate_write) {
+ ret = sigaction(SIGBUS, &oldact, NULL);
+ if (ret) {
+ /* Terminate QEMU since it can't recover from error */
+ perror("os_mem_prealloc: failed to reinstall signal handler");
+ exit(1);
+ }
}
}
--
2.31.1
