[PATCH v1 1/3] util/oslib-posix: Support MADV_POPULATE_WRITE for os_mem_prealloc()

[David Hildenbrand]

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 <david@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