shm: make max pool size user configurable (via a 'tweak' setting)

config.c

@@ -22,6 +22,7 @@
 #include "wayland.h"
 
 #define ALEN(v) (sizeof(v) / sizeof(v[0]))
+#define min(x, y) ((x) < (y) ? (x) : (y))
 
 static const uint32_t default_foreground = 0xdcdccc;
 static const uint32_t default_background = 0x111111;
@@ -621,6 +622,18 @@ parse_section_tweak(
         LOG_WARN("tweak: delayed-render-upper=%lu", ns);
     }
 
+    else if (strcmp(key, "max-shm-pool-size-mb") == 0) {
+        unsigned long mb;
+        if (!str_to_ulong(value, 10, &mb)) {
+            LOG_ERR("%s:%d: expected an integer: %s", path, lineno, value);
+            return false;
+        }
+
+        conf->tweak.max_shm_pool_size = min(mb * 1024 * 1024, INT32_MAX);
+        LOG_WARN("tweak: max-shm-pool-size=%lu bytes",
+                 conf->tweak.max_shm_pool_size);
+    }
+
     else {
         LOG_ERR("%s:%u: invalid key: %s", path, lineno, key);
         return false;
@@ -890,6 +903,7 @@ config_load(struct config *conf, const char *conf_path)
         .tweak = {
             .delayed_render_lower_ns = 500000,         /* 0.5ms */
             .delayed_render_upper_ns = 16666666 / 2,   /* half a frame period (60Hz) */
+            .max_shm_pool_size = 512 * 1024 * 1024,
         },
     };
 

config.h

@@ -77,6 +77,7 @@ struct config {
     struct {
         uint64_t delayed_render_lower_ns;
         uint64_t delayed_render_upper_ns;
+        off_t max_shm_pool_size;
     } tweak;
 };
 

main.c

@@ -330,6 +330,8 @@ main(int argc, char *const *argv)
         } while (errno == ERANGE);
     }
 
+    shm_set_max_pool_size(conf.tweak.max_shm_pool_size);
+
     if ((fdm = fdm_init()) == NULL)
         goto out;
 

shm.c

@@ -42,15 +42,23 @@
  *
  * On 32-bit the available address space is too small and SHM
  * scrolling is disabled.
+ *
+ * Note: this is the _default_ size. It can be overridden by calling
+ * shm_set_max_pool_size();
  */
-static const off_t max_pool_size = 512 * 1024 * 1024;
-//static const off_t max_pool_size = INT32_MAX;
+static off_t max_pool_size = 512 * 1024 * 1024;
 
 static tll(struct buffer) buffers;
 
 static bool can_punch_hole = false;
 static bool can_punch_hole_initialized = false;
 
+void
+shm_set_max_pool_size(off_t _max_pool_size)
+{
+    max_pool_size = _max_pool_size;
+}
+
 static void
 buffer_destroy_dont_close(struct buffer *buf)
 {
@@ -80,6 +88,15 @@ buffer_destroy(struct buffer *buf)
     buf->fd = -1;
 }
 
+void
+shm_fini(void)
+{
+    tll_foreach(buffers, it) {
+        buffer_destroy(&it->item);
+        tll_remove(buffers, it);
+    }
+}
+
 static void
 buffer_release(void *data, struct wl_buffer *wl_buffer)
 {
@@ -244,6 +261,8 @@ shm_get_buffer(struct wl_shm *shm, int width, int height, unsigned long cookie,
     off_t memfd_size = scrollable ? max_pool_size : size;
 #endif
 
+    LOG_DBG("memfd-size: %lu, initial offset: %lu", memfd_size, initial_offset);
+
     if (ftruncate(pool_fd, memfd_size) == -1) {
         LOG_ERRNO("failed to set size of SHM backing memory file");
         goto err;
@@ -333,15 +352,6 @@ err:
     return NULL;
 }
 
-void
-shm_fini(void)
-{
-    tll_foreach(buffers, it) {
-        buffer_destroy(&it->item);
-        tll_remove(buffers, it);
-    }
-}
-
 bool
 shm_can_scroll(const struct buffer *buf)
 {

shm.h

@@ -37,6 +37,7 @@ struct buffer *shm_get_buffer(
     struct wl_shm *shm, int width, int height, unsigned long cookie, bool scrollable);
 void shm_fini(void);
 
+void shm_set_max_pool_size(off_t max_pool_size);
 bool shm_can_scroll(const struct buffer *buf);
 bool shm_scroll(struct wl_shm *shm, struct buffer *buf, int rows,
                 int top_margin, int top_keep_rows,