[GC] Remove asserts for when calling madvise for huge pages fails. (#425)

The kernel doesn't always support MADV_HUGEPAGE or MADV_NOHUGEPAGE. In
particular, the man page for madvise says:

```
              The MADV_HUGEPAGE, MADV_NOHUGEPAGE, and MADV_COLLAPSE
              operations are available only if the kernel was configured
              with CONFIG_TRANSPARENT_HUGEPAGE
```

Since the madvise call doesn't have any material affect on the
allocator, it's just advice on how to organize the page tables as an
optimization for resident memory reduction, we can just ignore the
error.

Author: schveiguy

sdlib/d/gc/memmap.d

@@ -86,14 +86,68 @@ void pages_zero(void* addr, size_t size) {
 	}
 }
 
+/**
+ * Note about how we check the result of madvise in the following 2 functions.
+*
+ * From madvise(2):
+ *
+ *    The MADV_HUGEPAGE, MADV_NOHUGEPAGE, and MADV_COLLAPSE operations are
+ *    available only if the kernel was configured with
+ *    CONFIG_TRANSPARENT_HUGEPAGE
+ *
+ * This means that if a kernel is not configured with the right option, madvise
+ * will fail with EINVAL. The cases where EINVAL happens are listed as follows:
+ *
+ *     EINVAL addr is not page-aligned or size is negative.
+ *
+ *     EINVAL advice is not a valid.
+ *
+ *     EINVAL advice is MADV_COLD or MADV_PAGEOUT and the specified
+ *            address range includes locked, Huge TLB pages, or VM_PFNMAP
+ *            pages.
+ *
+ *     EINVAL advice is MADV_DONTNEED or MADV_REMOVE and the specified
+ *            address range includes locked, Huge TLB pages, or VM_PFNMAP
+ *            pages.
+ *
+ *     EINVAL advice is MADV_MERGEABLE or MADV_UNMERGEABLE, but the
+ *            kernel was not configured with CONFIG_KSM.
+ *
+ *     EINVAL advice is MADV_FREE or MADV_WIPEONFORK but the specified
+ *            address range includes file, Huge TLB, MAP_SHARED, or
+ *            VM_PFNMAP ranges.
+ *
+ *     EINVAL advice is MADV_POPULATE_READ or MADV_POPULATE_WRITE, but
+ *            the specified address range includes ranges with
+ *            insufficient permissions or special mappings, for example,
+ *            mappings marked with kernel-internal flags such a VM_IO or
+ *            VM_PFNMAP, or secret memory regions created using
+ *            memfd_secret(2).
+ *
+ *     EINVAL advice is MADV_GUARD_INSTALL or MADV_GUARD_REMOVE, but the
+ *            specified address range contains an unsupported mapping.
+ *
+ * We do not have to worry about anything other than the first two. For a
+ * kernel without the right configuration, EINVAL will happen for MADV_HUGEPAGE
+ * and MADV_NOHUGEPAGE, because the advice is not valid. But we should also
+ * check for the first case, that the page is not aligned or the size is
+ * negative (which is odd, since size_t is unsigned, even in C).
+ */
+
 void pages_hugify(void* addr, size_t size) {
+	assert(isAligned(addr, PageSize), "Not aligned!");
+	assert(cast(long) size > 0, "Negative size!");
 	auto ret = madvise(addr, size, Madv.HugePage);
-	assert(ret == 0, "madvise failed!");
+	// See note above.
+	// assert(ret == 0, "madvise failed!");
 }
 
 void pages_dehugify(void* addr, size_t size) {
+	assert(isAligned(addr, PageSize), "Not aligned!");
+	assert(cast(long) size > 0, "Negative size!");
 	auto ret = madvise(addr, size, Madv.NoHugePage);
-	assert(ret == 0, "madvise failed!");
+	// See note above.
+	//assert(ret == 0, "madvise failed!");
 }
 
 private: