Re: [Qemu-devel] [PATCH V13 4/5] util/mmap-alloc: support MAP_SYNC in qemu_ram_mmap()

[Eduardo Habkost]

On Thu, Apr 18, 2019 at 07:05:16PM -0300, Eduardo Habkost wrote:
> Hi,
> 
> I found out that this series missed QEMU 4.0 and I was going to
> queue for 4.1, but unfortunately this patch conflicts with:
> 
> commit 2044c3e7116eeac0449dcb4a4130cc8f8b9310da
> Author: Murilo Opsfelder Araujo <address@hidden>
> Date:   Wed Jan 30 21:36:04 2019 -0200
> 
>     mmap-alloc: unfold qemu_ram_mmap()
[...]
> On Fri, Feb 08, 2019 at 06:11:11PM +0800, Zhang, Yi wrote:
> > From: Zhang Yi <address@hidden>
> > 
> > When a file supporting DAX is used as vNVDIMM backend, mmap it with
> > MAP_SYNC flag in addition which can ensure file system metadata
> > synced in each guest writes to the backend file, without other QEMU
> > actions (e.g., periodic fsync() by QEMU).
> > 
> > Current, We have below different possible use cases:
> > 
> > 1. pmem=on is set, shared=on is set, MAP_SYNC supported:
> >    a: backend is a dax supporting file.
> >     - MAP_SYNC will active.
> >    b: backend is not a dax supporting file.
> >     - mmap will trigger a warning. then MAP_SYNC flag will be ignored
> > 
> > 2. The rest of cases:
> >    - we will never pass the MAP_SYNC to mmap2
> > 
> > Signed-off-by: Haozhong Zhang <address@hidden>
> > Signed-off-by: Zhang Yi <address@hidden>

Untested patch rebase is below.  Can Intel help test it and
submit v14?

Signed-off-by: Haozhong Zhang <address@hidden>
Signed-off-by: Zhang Yi <address@hidden>
[ehabkost: Rebased patch to latest code on master]
Signed-off-by: Eduardo Habkost <address@hidden>
---
 util/mmap-alloc.c | 41 +++++++++++++++++++++++++++++++++++++++--
 1 file changed, 39 insertions(+), 2 deletions(-)

diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c
index 9713f4b960..7063a44c7d 100644
--- a/util/mmap-alloc.c
+++ b/util/mmap-alloc.c
@@ -10,6 +10,13 @@
  * later.  See the COPYING file in the top-level directory.
  */
 
+#ifdef CONFIG_LINUX
+#include <linux/mman.h>
+#else  /* !CONFIG_LINUX */
+#define MAP_SYNC              0x0
+#define MAP_SHARED_VALIDATE   0x0
+#endif /* CONFIG_LINUX */
+
 #include "qemu/osdep.h"
 #include "qemu/mmap-alloc.h"
 #include "qemu/host-utils.h"
@@ -82,6 +89,7 @@ void *qemu_ram_mmap(int fd,
                     bool is_pmem)
 {
     int flags;
+    int map_sync_flags = 0;
     int guardfd;
     size_t offset;
     size_t pagesize;
@@ -132,10 +140,39 @@ void *qemu_ram_mmap(int fd,
     flags = MAP_FIXED;
     flags |= fd == -1 ? MAP_ANONYMOUS : 0;
     flags |= shared ? MAP_SHARED : MAP_PRIVATE;
+    if (shared && is_pmem) {
+        map_sync_flags = MAP_SYNC | MAP_SHARED_VALIDATE;
+    }
     offset = QEMU_ALIGN_UP((uintptr_t)guardptr, align) - (uintptr_t)guardptr;
 
-    ptr = mmap(guardptr + offset, size, PROT_READ | PROT_WRITE, flags, fd, 0);
-
+    ptr = mmap(guardptr + offset, size, PROT_READ | PROT_WRITE,
+               flags | map_sync_flags, fd, 0);
+
+    if (ptr == MAP_FAILED && map_sync_flags) {
+        if (errno == ENOTSUP) {
+            char *proc_link, *file_name;
+            int len;
+            proc_link = g_strdup_printf("/proc/self/fd/%d", fd);
+            file_name = g_malloc0(PATH_MAX);
+            len = readlink(proc_link, file_name, PATH_MAX - 1);
+            if (len < 0) {
+                len = 0;
+            }
+            file_name[len] = '\0';
+            fprintf(stderr, "Warning: requesting persistence across crashes "
+                    "for backend file %s failed. Proceeding without "
+                    "persistence, data might become corrupted in case of host "
+                    "crash.\n", file_name);
+            g_free(proc_link);
+            g_free(file_name);
+        }
+        /*
+         * if map failed with MAP_SHARED_VALIDATE | MAP_SYNC,
+         * we will remove these flags to handle compatibility.
+         */
+        ptr = mmap(guardptr + offset, size, PROT_READ | PROT_WRITE,
+                   flags, fd, 0);
+    }
     if (ptr == MAP_FAILED) {
         munmap(guardptr, total);
         return MAP_FAILED;
-- 
2.18.0.rc1.1.g3f1ff2140