Increase SEG_ALLOC_SIZE_MAX to 64MB to support OPcache JIT on Solaris

[psumbera]

The SysV shared memory allocator in OPcache hardcodes a maximum segment size of 32MB (SEG_ALLOC_SIZE_MAX). With JIT enabled, OPcache reserves 64MB (ZEND_JIT_DEFAULT_BUFFER_SIZE) from the last segment, causing startup to fail with "Insufficient shared memory!".

This patch increases SEG_ALLOC_SIZE_MAX to 64MB so the reserved JIT buffer fits in a single segment. Behavior on other platforms using mmap remains unaffected.

Fixes #20718.

[iluuu1994]

Won't this issue just immediately re-appear when configuring a larger buffer size? E.g. add -d opcache.jit_buffer_size=128M and it's back.

[iluuu1994]

PHP 8.3 has the same issue, it's just enabled in a different way (opcache.jit_buffer_size has to be set).

[iluuu1994]

So, I think the maximum segment size should be bumped way up, at least on 64-bit architectures. Alternatively, we can request a bigger segment size only when JIT is enabled (though I don't really see a benefit over just always allocating bigger chunks). Something like:

diff --git a/ext/opcache/shared_alloc_shm.c b/ext/opcache/shared_alloc_shm.c
index 09a357d189e..2f7c9ba4217 100644
--- a/ext/opcache/shared_alloc_shm.c
+++ b/ext/opcache/shared_alloc_shm.c
@@ -20,6 +20,9 @@
 */
 
 #include "zend_shared_alloc.h"
+#ifdef HAVE_JIT
+# include "jit/zend_jit.h"
+#endif
 
 #ifdef USE_SHM
 
@@ -50,6 +53,23 @@ typedef struct  {
     int shm_id;
 } zend_shared_segment_shm;
 
+#ifdef HAVE_JIT
+static size_t ceil_to_power_of_2(size_t n)
+{
+	ZEND_ASSERT(n);
+
+	size_t m = n;
+	uint32_t r = 0;
+	while (m >>= 1) {
+		r++;
+	}
+	if (n != (((size_t)1) << r)) {
+		r++;
+	}
+	return ((size_t)1) << r;
+}
+#endif
+
 static int create_segments(size_t requested_size, zend_shared_segment_shm ***shared_segments_p, int *shared_segments_count, const char **error_in)
 {
 	int i;
@@ -61,6 +81,12 @@ static int create_segments(size_t requested_size, zend_shared_segment_shm ***sha
 	zend_shared_segment_shm *shared_segments;
 
 	seg_allocate_size = SEG_ALLOC_SIZE_MAX;
+#ifdef HAVE_JIT
+	if (JIT_G(on) && JIT_G(buffer_size) > seg_allocate_size) {
+		/* The JIT buffer must be allocated in a contiguous chunk. */
+		seg_allocate_size = ceil_to_power_of_2(JIT_G(buffer_size));
+	}
+#endif
 	/* determine segment size we _really_ need:
 	 * no more than to include requested_size
 	 */