[Tarantool-patches] [PATCH luajit 2/2] Linux/ARM64: Make mremap() non-moving due to VA space woes.

[Sergey Kaplun]

From: Mike Pall <mike>

This reduces overall performance on ARM64, but we have no choice.
Linux kernel default userspace VA is 48 bit, but we'd need 47 bit.
mremap() ignores address hints due to a kernel API issue. The mapping
may move to an undesired address which will cause an assert or crash.

Reported by Raymond W. Ko.

(cherry picked from commit 67dbec82f4f05a416a78a560a726553beaa7a223)

47-bit VA space is required by LuaJIT for keeping a GC object pointer in
TValue. When need to reallocate to huge sized block `mrepmap()` on arm64
may move out VA space from the 47-bit range. `mremap()` accepts the
fifth argument (new address hint) only with MREMAP_FIXED flag. In that
case it unmaps any other mapping to specified address.

To avoid this behaviour this patch restricts `mremap()` to relocate
the mapping to a new virtual address by reset MREMAP_MAYMOVE flag
for arm64 architecture.

Sergey Kaplun:
* added the description and the test for the problem

Needed for tarantool/tarantool#6154
---
 src/lj_alloc.c                                |  2 +-
 .../lj-671-arm64-assert-after-mremap.test.lua | 24 +++++++++++++++++++
 2 files changed, 25 insertions(+), 1 deletion(-)
 create mode 100644 test/tarantool-tests/lj-671-arm64-assert-after-mremap.test.lua

diff --git a/src/lj_alloc.c b/src/lj_alloc.c
index 9fc761c7..ffcd019b 100644
--- a/src/lj_alloc.c
+++ b/src/lj_alloc.c
@@ -378,7 +378,7 @@ static void *CALL_MREMAP_(void *ptr, size_t osz, size_t nsz, int flags)
 #define CALL_MREMAP(addr, osz, nsz, mv) CALL_MREMAP_((addr), (osz), (nsz), (mv))
 #define CALL_MREMAP_NOMOVE	0
 #define CALL_MREMAP_MAYMOVE	1
-#if LJ_64 && !LJ_GC64
+#if LJ_64 && (!LJ_GC64 || LJ_TARGET_ARM64)
 #define CALL_MREMAP_MV		CALL_MREMAP_NOMOVE
 #else
 #define CALL_MREMAP_MV		CALL_MREMAP_MAYMOVE
diff --git a/test/tarantool-tests/lj-671-arm64-assert-after-mremap.test.lua b/test/tarantool-tests/lj-671-arm64-assert-after-mremap.test.lua
new file mode 100644
index 00000000..0be60a2d
--- /dev/null
+++ b/test/tarantool-tests/lj-671-arm64-assert-after-mremap.test.lua
@@ -0,0 +1,24 @@
+local tap = require('tap')
+
+-- Test file to demonstrate assertion after `mremap()` on arm64.
+-- See also, https://github.com/LuaJIT/LuaJIT/issues/671.
+
+local test = tap.test('lj-671-arm64-assert-after-mremap')
+test:plan(1)
+
+-- `mremap()` is used on Linux for remap directly mapped big
+-- (>=DEFAULT_MMAP_THRESHOLD) memory chunks.
+-- The simplest way to test memory move is to allocate the huge
+-- memory chunk for string buffer directly and reallocate it
+-- after.
+-- To allocate buffer exactly to threshold limit for direct chunk
+-- mapping use `string.rep()` with length equals threshold.
+-- Then concatenate result string (with length of
+-- DEFAULT_MMAP_THRESHOLD) with the other one to reallocate
+-- and remap string buffer.
+
+local DEFAULT_MMAP_THRESHOLD = 128 * 1024
+local s = string.rep('x', DEFAULT_MMAP_THRESHOLD)..'x'
+test:ok(s)
+
+os.exit(test:check() and 0 or 1)
-- 
2.31.0