diff --git a/mdbx.h b/mdbx.h
index c4712c73..09143488 100644
--- a/mdbx.h
+++ b/mdbx.h
@@ -1684,6 +1684,7 @@ LIBMDBX_API MDBX_oom_func *mdbx_env_get_oomfunc(MDBX_env *env);
 #define MDBX_DBG_AUDIT 16
 #define MDBX_DBG_JITTER 32
 #define MDBX_DBG_DUMP 64
+#define MDBX_DBG_LEGACY_MULTIOPEN 128
 
 typedef void MDBX_debug_func(int type, const char *function, int line,
                              const char *msg, va_list args);
diff --git a/src/lck-linux.c b/src/lck-linux.c
index e9e48b04..151ba325 100644
--- a/src/lck-linux.c
+++ b/src/lck-linux.c
@@ -37,12 +37,10 @@
 #endif
 #endif /* MDBX_USE_ROBUST */
 
-uint32_t linux_kernel_version;
-static int op_setlk = F_SETLK, op_setlkw = F_SETLKW, op_getlk = F_GETLK;
-
 /*----------------------------------------------------------------------------*/
 /* global constructor/destructor */
 
+uint32_t mdbx_linux_kernel_version;
 static __cold __attribute__((constructor)) void mdbx_global_constructor(void) {
   struct utsname buffer;
   if (uname(&buffer) == 0) {
@@ -54,7 +52,7 @@ static __cold __attribute__((constructor)) void mdbx_global_constructor(void) {
         if (number > 0) {
           if (number > 255)
             number = 255;
-          linux_kernel_version += number << (24 - i * 8);
+          mdbx_linux_kernel_version += number << (24 - i * 8);
         }
         ++i;
       } else {
@@ -63,16 +61,6 @@ static __cold __attribute__((constructor)) void mdbx_global_constructor(void) {
     }
   }
 
-#if defined(F_OFD_SETLK) && defined(F_OFD_SETLKW) && defined(F_OFD_GETLK)
-  if (linux_kernel_version >
-      0x030f0000 /* OFD locks are available since 3.15, but engages here only
-        for 3.16 and larer kernels (LTS) for reliability reasons */) {
-    op_setlk = F_OFD_SETLK;
-    op_setlkw = F_OFD_SETLKW;
-    op_getlk = F_OFD_GETLK;
-  }
-#endif /* OFD locks */
-
   mdbx_rthc_global_init();
 }
 
@@ -118,6 +106,25 @@ static __cold __attribute__((destructor)) void mdbx_global_destructor(void) {
  *    и атомарным режимами блокировок.
  */
 
+static int op_setlk, op_setlkw, op_getlk;
+static void __cold choice_fcntl() {
+  assert(!op_setlk && !op_setlkw && !op_getlk);
+#if defined(F_OFD_SETLK) && defined(F_OFD_SETLKW) && defined(F_OFD_GETLK)
+  if (mdbx_linux_kernel_version >
+          0x030f0000 /* OFD locks are available since 3.15, but engages here
+          only for 3.16 and larer kernels (LTS) for reliability reasons */
+      && (mdbx_runtime_flags & MDBX_DBG_LEGACY_MULTIOPEN) == 0) {
+    op_setlk = F_OFD_SETLK;
+    op_setlkw = F_OFD_SETLKW;
+    op_getlk = F_OFD_GETLK;
+    return;
+  }
+#endif /* OFD locks */
+  op_setlk = F_SETLK;
+  op_setlkw = F_SETLKW;
+  op_getlk = F_GETLK;
+}
+
 #ifndef OFF_T_MAX
 #define OFF_T_MAX                                                              \
   ((sizeof(off_t) > 4 ? INT64_MAX : INT32_MAX) & ~(size_t)0xffff)
@@ -341,6 +348,8 @@ static int __cold internal_seize_lck(int lfd) {
 
 int __cold mdbx_lck_seize(MDBX_env *env) {
   assert(env->me_fd != INVALID_HANDLE_VALUE);
+  if (unlikely(op_setlk == 0))
+    choice_fcntl();
 
   if (env->me_lfd == INVALID_HANDLE_VALUE) {
     /* LY: without-lck mode (e.g. exclusive or on read-only filesystem) */
diff --git a/src/mdbx.c b/src/mdbx.c
index 61098380..d44b47b5 100644
--- a/src/mdbx.c
+++ b/src/mdbx.c
@@ -12917,36 +12917,51 @@ int __cold mdbx_reader_check0(MDBX_env *env, int rdt_locked, int *dead) {
 }
 
 int __cold mdbx_setup_debug(int flags, MDBX_debug_func *logger) {
-  unsigned ret = mdbx_runtime_flags;
-  mdbx_runtime_flags = flags;
-
+  const int rc = mdbx_runtime_flags;
+  if (flags != -1) {
+#if MDBX_DEBUG
+    flags &= MDBX_DBG_DUMP | MDBX_DBG_LEGACY_MULTIOPEN;
+#else
+    flags &= MDBX_DBG_ASSERT | MDBX_DBG_PRINT | MDBX_DBG_TRACE |
+             MDBX_DBG_EXTRA | MDBX_DBG_AUDIT | MDBX_DBG_JITTER | MDBX_DBG_DUMP |
+             MDBX_DBG_LEGACY_MULTIOPEN;
+#endif
 #if defined(__linux__) || defined(__gnu_linux__)
-  if (flags & MDBX_DBG_DUMP) {
-    int core_filter_fd = open("/proc/self/coredump_filter", O_TRUNC | O_RDWR);
-    if (core_filter_fd >= 0) {
-      char buf[32];
-      const unsigned r = pread(core_filter_fd, buf, sizeof(buf), 0);
-      if (r > 0 && r < sizeof(buf)) {
-        buf[r] = 0;
-        unsigned long mask = strtoul(buf, NULL, 16);
-        if (mask != ULONG_MAX) {
-          mask |= 1 << 3 /* Dump file-backed shared mappings */;
-          mask |= 1 << 6 /* Dump shared huge pages */;
-          mask |= 1 << 8 /* Dump shared DAX pages */;
-          unsigned w = snprintf(buf, sizeof(buf), "0x%lx\n", mask);
-          if (w > 0 && w < sizeof(buf)) {
-            w = pwrite(core_filter_fd, buf, w, 0);
-            (void)w;
+    if ((mdbx_runtime_flags ^ flags) & MDBX_DBG_DUMP) {
+      /* http://man7.org/linux/man-pages/man5/core.5.html */
+      const unsigned long dump_bits =
+          1 << 3   /* Dump file-backed shared mappings */
+          | 1 << 6 /* Dump shared huge pages */
+          | 1 << 8 /* Dump shared DAX pages */;
+      const int core_filter_fd =
+          open("/proc/self/coredump_filter", O_TRUNC | O_RDWR);
+      if (core_filter_fd != -1) {
+        char buf[32];
+        intptr_t bytes = pread(core_filter_fd, buf, sizeof(buf), 0);
+        if (bytes > 0 && (size_t)bytes < sizeof(buf)) {
+          buf[bytes] = 0;
+          const unsigned long present_mask = strtoul(buf, NULL, 16);
+          const unsigned long wanna_mask = (flags & MDBX_DBG_DUMP)
+                                               ? present_mask | dump_bits
+                                               : present_mask & ~dump_bits;
+          if (wanna_mask != present_mask) {
+            bytes = snprintf(buf, sizeof(buf), "0x%lx\n", wanna_mask);
+            if (bytes > 0 && (size_t)bytes < sizeof(buf)) {
+              bytes = pwrite(core_filter_fd, buf, bytes, 0);
+              (void)bytes;
+            }
           }
         }
+        close(core_filter_fd);
       }
-      close(core_filter_fd);
     }
-  }
 #endif /* Linux */
+    mdbx_runtime_flags = flags;
+  }
 
-  mdbx_debug_logger = logger;
-  return ret;
+  if (-1 != (intptr_t)logger)
+    mdbx_debug_logger = logger;
+  return rc;
 }
 
 static txnid_t __cold mdbx_oomkick(MDBX_env *env, const txnid_t laggard) {
diff --git a/src/osal.c b/src/osal.c
index e0ae3f47..8fb23c59 100644
--- a/src/osal.c
+++ b/src/osal.c
@@ -674,7 +674,7 @@ int mdbx_filesync(mdbx_filehandle_t fd, enum mdbx_syncmode_bits mode_bits) {
     return likely(fcntl(fd, F_FULLFSYNC) != -1) ? MDBX_SUCCESS : errno;
 #endif /* MacOS */
 #if defined(__linux__) || defined(__gnu_linux__)
-  if (mode_bits == MDBX_SYNC_SIZE && linux_kernel_version >= 0x03060000)
+  if (mode_bits == MDBX_SYNC_SIZE && mdbx_linux_kernel_version >= 0x03060000)
     return MDBX_SUCCESS;
 #endif /* Linux */
   int rc;
@@ -789,7 +789,7 @@ int mdbx_msync(mdbx_mmap_t *map, size_t offset, size_t length, int async) {
   return GetLastError();
 #else
 #ifdef __linux__
-  if (async && linux_kernel_version > 0x02061300)
+  if (async && mdbx_linux_kernel_version > 0x02061300)
     /* Since Linux 2.6.19, MS_ASYNC is in fact a no-op,
        since the kernel properly tracks dirty pages and flushes them to storage
        as necessary. */
diff --git a/src/osal.h b/src/osal.h
index 90189762..45d4cf39 100644
--- a/src/osal.h
+++ b/src/osal.h
@@ -475,7 +475,7 @@ int mdbx_vasprintf(char **strp, const char *fmt, va_list ap);
 #define MAX_WRITE UINT32_C(0x3fff0000)
 
 #if defined(__linux__) || defined(__gnu_linux__)
-extern uint32_t linux_kernel_version;
+extern uint32_t mdbx_linux_kernel_version;
 #endif /* Linux */
 
 /* Get the size of a memory page for the system.