diff --git a/src/libslic3r/utils.cpp b/src/libslic3r/utils.cpp
index d785a71a..03440bd6 100644
--- a/src/libslic3r/utils.cpp
+++ b/src/libslic3r/utils.cpp
@@ -1591,11 +1591,26 @@ size_t available_physical_memory()
     return (size_t) status.ullAvailPhys;
 
 #elif defined(__linux__)
-    struct sysinfo info;
-    if (sysinfo(&info) == 0) {
-        return info.freeram * info.mem_unit;
+    // Prefer MemAvailable (free + reclaimable page/buffer cache) from
+    // /proc/meminfo. sysinfo().freeram is only MemFree, which Linux keeps
+    // low by design (idle RAM is used for cache), so it grossly understates
+    // usable memory and must not be treated as "available".
+    if (FILE* mi = fopen("/proc/meminfo", "r")) {
+        char line[256];
+        unsigned long long kb = 0;
+        bool found = false;
+        while (fgets(line, sizeof(line), mi) != nullptr) {
+            if (sscanf(line, "MemAvailable: %llu kB", &kb) == 1) { found = true; break; }
+        }
+        fclose(mi);
+        if (found)
+            return (size_t)kb * (size_t)1024;
     }
-	return 0L;
+    // Fallback for kernels < 3.14 without MemAvailable: free + buffers.
+    struct sysinfo info;
+    if (sysinfo(&info) == 0)
+        return (size_t)(info.freeram + info.bufferram) * (size_t)info.mem_unit;
+    return 0L;
 #elif defined(__APPLE__)
     int      mib[2] = {CTL_HW, HW_MEMSIZE};
     uint64_t memsize;
diff --git a/src/slic3r/GUI/GCodeRenderer/AdvancedRenderer.cpp b/src/slic3r/GUI/GCodeRenderer/AdvancedRenderer.cpp
index a5133ae4..92c77b7e 100644
--- a/src/slic3r/GUI/GCodeRenderer/AdvancedRenderer.cpp
+++ b/src/slic3r/GUI/GCodeRenderer/AdvancedRenderer.cpp
@@ -748,20 +748,12 @@ namespace Slic3r
 				// Import gcode file, preview using normal mode (no lite mode)
                 bool is_lite_mode_cfg = (wxGetApp().app_config->get_bool("gcode_preview_lite_mode") && (!m_only_gcode_in_preview));
 
-                // Auto enable lite mode on Linux if memory is tight or preview is huge.
-#if defined(__linux__)
-                {
-                    size_t       avail_bytes           = Slic3r::available_physical_memory();
-                    size_t       total_bytes           = Slic3r::total_physical_memory();
-                    const size_t moves_count_local     = gcode_result.moves.size();
-                    const size_t avail_threshold_bytes = size_t(2) * size_t(1024) * size_t(1024) * size_t(1024); // 2 GB
-                    const bool   low_available         = (avail_bytes > 0 && avail_bytes < avail_threshold_bytes);
-                    const bool   low_ratio    = (total_bytes > 0 && avail_bytes > 0 && (double) avail_bytes / (double) total_bytes < 0.125);
-                    const bool   huge_preview = (moves_count_local > 2000000);
-                    if (!m_only_gcode_in_preview && (low_available || low_ratio || huge_preview))
-                        is_lite_mode_cfg = true;
-                }
-#endif
+                // NOTE (#512): Removed the Linux-only auto-enable-lite-mode heuristic.
+                // It force-enabled lite mode against the user's explicit
+                // gcode_preview_lite_mode=false setting, and tripped on broken memory
+                // readings (available/total reported as 0 on some systems), permanently
+                // hiding infill in the preview on Linux. Lite mode is now strictly
+                // driven by the gcode_preview_lite_mode config value above.
 
                 const bool is_lite_mode = m_is_lite_mode = is_lite_mode_cfg;
 
diff --git a/src/slic3r/GUI/GUI_Preview.cpp b/src/slic3r/GUI/GUI_Preview.cpp
index 2b6e5420..47f2ca5e 100644
--- a/src/slic3r/GUI/GUI_Preview.cpp
+++ b/src/slic3r/GUI/GUI_Preview.cpp
@@ -742,31 +742,11 @@ void Preview::load_print_as_fff(bool keep_z_range, bool only_gcode)
             // import gcode file, preview using normal mode (no lite mode)
             bool is_lite_mode = wxGetApp().app_config->get_bool("gcode_preview_lite_mode") && (!only_gcode);
 
-            // Auto enable lite mode on Linux if memory is tight or preview is huge
-            // This does NOT affect Windows and does not persist the setting.
-#if defined(__linux__)
-            {
-                size_t avail_bytes = Slic3r::available_physical_memory();
-                size_t total_bytes = Slic3r::total_physical_memory();
-                const size_t moves_count = m_gcode_result ? m_gcode_result->moves.size() : 0;
-
-                // Heuristics:
-                // - Consider memory tight if available < 2 GB or available < 12.5% of total.
-                // - Consider preview huge if moves exceed 2 million.
-                const size_t avail_threshold_bytes = size_t(2) * size_t(1024) * size_t(1024) * size_t(1024); // 2 GB
-                const bool low_available = (avail_bytes > 0 && avail_bytes < avail_threshold_bytes);
-                const bool low_ratio     = (total_bytes > 0 && avail_bytes > 0 && (double)avail_bytes / (double)total_bytes < 0.125);
-                const bool huge_preview  = (moves_count > 2000000);
-
-                if (!only_gcode && (low_available || low_ratio || huge_preview)) {
-                    is_lite_mode = true;
-                    // Also enable shell-only surface rendering to further reduce load.
-                    if (m_gcode_result) {
-                        m_gcode_result->all_surface_with_shell = true;
-                    }
-                }
-            }
-#endif
+            // NOTE (#512): removed the Linux auto-enable-lite-mode heuristic
+            // here (mirrors the advanced renderer). It overrode the user's
+            // explicit gcode_preview_lite_mode setting and, because
+            // available_physical_memory() reported MemFree, fired on almost
+            // every Linux machine. Lite mode now follows the config value above.
             std::vector<std::pair<EMoveType, size_t>> changed_tmp;
             GCodeProcessorResult* tmp_result = nullptr;
             if (is_lite_mode) {