[PATCH v2 3/5] ntdll: Add MADV_DONTNEED-based fast path for NtFlushProcessWriteBuffers.

[Torge Matthies]

From: Torge Matthies

Credits to Avi Kivity (scylladb) and Aliaksei Kandratsenka (gperftools) for this trick, see [1].

[1] https://github.com/scylladb/seastar/commit/77a58e4dc020233f66fccb8d9e8f7a8b7...
---
 dlls/ntdll/unix/virtual.c | 54 ++++++++++++++++++++++++++++++++++++---
 1 file changed, 50 insertions(+), 4 deletions(-)

diff --git a/dlls/ntdll/unix/virtual.c b/dlls/ntdll/unix/virtual.c
index 09fb8eaf4a6..eba6be62337 100644
--- a/dlls/ntdll/unix/virtual.c
+++ b/dlls/ntdll/unix/virtual.c
@@ -214,6 +214,11 @@ struct range_entry
 static struct range_entry *free_ranges;
 static struct range_entry *free_ranges_end;

+#if defined(__linux__) && (defined(__i386__) || defined(__x86_64__))
+static void *dontneed_page;
+static pthread_mutex_t dontneed_page_mutex = PTHREAD_MUTEX_INITIALIZER;
+#endif
+

static inline BOOL is_beyond_limit( const void *addr, size_t size, const void *limit )
 {
@@ -5015,10 +5020,40 @@ NTSTATUS WINAPI NtFlushInstructionCache( HANDLE handle, const void *addr, SIZE_T
 }

-/**********************************************************************
- *           NtFlushProcessWriteBuffers  (NTDLL.@)
- */
-void WINAPI NtFlushProcessWriteBuffers(void)
+#if defined(__linux__) && (defined(__i386__) || defined(__x86_64__))
+static int try_madvise( void )
+{
+    int ret = 0;
+    char *mem;
+
+    pthread_mutex_lock(&dontneed_page_mutex);
+    /* Credits to Avi Kivity (scylladb) and Aliaksei Kandratsenka (gperftools) for this trick,
+     * see https://github.com/scylladb/seastar/commit/77a58e4dc020233f66fccb8d9e8f7a8b7... */
+    mem = dontneed_page;
+    if (!mem)
+    {
+        mem = anon_mmap_alloc( page_size, PROT_READ | PROT_WRITE );
+        if (mem == MAP_FAILED)
+            goto failed;
+        if (mlock( mem, page_size ))
+        {
+            munmap( mem, page_size );
+            goto failed;
+        }
+        dontneed_page = mem;
+    }
+    *mem = 3;
+    ret = !madvise( mem, page_size, MADV_DONTNEED );
+failed:
+    pthread_mutex_unlock(&dontneed_page_mutex);
+    return ret;
+}
+#else
+static int try_madvise( void ) { return 0; }
+#endif
+
+
+static void do_apc_memorybarrier( void )
 {
     NTSTATUS status;

@@ -5043,6 +5078,17 @@ void WINAPI NtFlushProcessWriteBuffers(void)
 }

+/**********************************************************************
+ *           NtFlushProcessWriteBuffers  (NTDLL.@)
+ */
+void WINAPI NtFlushProcessWriteBuffers(void)
+{
+    if (try_madvise())
+        return;
+    do_apc_memorybarrier();
+}
+
+
 /**********************************************************************
  *           NtCreatePagingFile  (NTDLL.@)
  */