Enable guard pages in CHECK_CLIENT

Change the behaviour to use PROT_NONE for reservations in CHECK_CLIENT
mode.  This means that we only provide access once data is actually
being used.

src/pal/pal_apple.h

@@ -113,7 +113,7 @@ namespace snmalloc
     {
       SNMALLOC_ASSERT(is_aligned_block<page_size>(p, size));
 
-#  ifdef USE_POSIX_COMMIT_CHECKS
+#  if defined(SNMALLOC_CHECK_CLIENT) && !defined(NDEBUG)
       memset(p, 0x5a, size);
 #  endif
 
@@ -126,7 +126,7 @@ namespace snmalloc
       while (madvise(p, size, MADV_FREE_REUSABLE) == -1 && errno == EAGAIN)
         ;
 
-#  ifdef USE_POSIX_COMMIT_CHECKS
+#  ifdef SNMALLOC_CHECK_CLIENT
       // This must occur after `MADV_FREE_REUSABLE`.
       //
       // `mach_vm_protect` is observably slower in benchmarks.
@@ -179,7 +179,7 @@ namespace snmalloc
         }
       }
 
-#  ifdef USE_POSIX_COMMIT_CHECKS
+#  ifdef SNMALLOC_CHECK_CLIENT
       // Mark pages as writable for `madvise` below.
       //
       // `mach_vm_protect` is observably slower in benchmarks.
@@ -218,7 +218,7 @@ namespace snmalloc
       // must be initialized to 0 or addr is interepreted as a lower-bound.
       mach_vm_address_t addr = 0;
 
-#  ifdef USE_POSIX_COMMIT_CHECKS
+#  ifdef SNMALLOC_CHECK_CLIENT
       vm_prot_t prot = committed ? VM_PROT_READ | VM_PROT_WRITE : VM_PROT_NONE;
 #  else
       vm_prot_t prot = VM_PROT_READ | VM_PROT_WRITE;

src/pal/pal_bsd.h

@@ -35,11 +35,11 @@ namespace snmalloc
     {
       SNMALLOC_ASSERT(is_aligned_block<OS::page_size>(p, size));
       // Call this Pal to simulate the Windows decommit in CI.
-#ifdef USE_POSIX_COMMIT_CHECKS
+#if defined(SNMALLOC_CHECK_CLIENT) && !defined(NDEBUG)
       memset(p, 0x5a, size);
 #endif
       madvise(p, size, MADV_FREE);
-#ifdef USE_POSIX_COMMIT_CHECKS
+#ifdef SNMALLOC_CHECK_CLIENT
       mprotect(p, size, PROT_NONE);
 #endif
     }

src/pal/pal_bsd_aligned.h

@@ -37,10 +37,16 @@ namespace snmalloc
 
       int log2align = static_cast<int>(bits::next_pow2_bits(size));
 
+#ifdef SNMALLOC_CHECK_CLIENT
+      auto prot = committed ? PROT_READ | PROT_WRITE : PROT_NONE;
+#else
+      auto prot = PROT_READ | PROT_WRITE;
+#endif
+
       void* p = mmap(
         nullptr,
         size,
-        PROT_READ | PROT_WRITE,
+        prot,
         MAP_PRIVATE | MAP_ANONYMOUS | MAP_ALIGNED(log2align),
         -1,
         0);

src/pal/pal_posix.h

@@ -165,10 +165,12 @@ namespace snmalloc
     static void notify_not_using(void* p, size_t size) noexcept
     {
       SNMALLOC_ASSERT(is_aligned_block<OS::page_size>(p, size));
-#ifdef USE_POSIX_COMMIT_CHECKS
+#ifdef SNMALLOC_CHECK_CLIENT
       // Fill memory so that when we switch the pages back on we don't make
       // assumptions on the content.
+#  if !defined(NDEBUG)
       memset(p, 0x5a, size);
+#  endif
       mprotect(p, size, PROT_NONE);
 #else
       UNUSED(p);
@@ -189,7 +191,7 @@ namespace snmalloc
       SNMALLOC_ASSERT(
         is_aligned_block<OS::page_size>(p, size) || (zero_mem == NoZero));
 
-#ifdef USE_POSIX_COMMIT_CHECKS
+#ifdef SNMALLOC_CHECK_CLIENT
       mprotect(p, size, PROT_READ | PROT_WRITE);
 #else
       UNUSED(p);
@@ -260,6 +262,12 @@ namespace snmalloc
       constexpr size_t min_size =
         bits::is64() ? bits::one_at_bit(31) : bits::one_at_bit(27);
 
+#ifdef SNMALLOC_CHECK_CLIENT
+      auto prot = PROT_NONE;
+#else
+      auto prot = PROT_READ | PROT_WRITE;
+#endif
+
       for (size_t size_request = bits::max(size, min_size);
            size_request >= size;
            size_request = size_request / 2)
@@ -267,7 +275,7 @@ namespace snmalloc
         void* p = mmap(
           nullptr,
           size_request,
-          PROT_READ | PROT_WRITE,
+          prot,
           MAP_PRIVATE | MAP_ANONYMOUS | DefaultMMAPFlags<OS>::flags,
           AnonFD<OS>::fd,
           0);