hash: prevent eleventy-gigabyte core dumps

Add MADV_DONTDUMP to the list of advice flags.

There are now three flags which may or may not be supported by the
target kernel.  Try each one and log its success or failure separately.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

src/bees-hash.cc

@@ -635,12 +635,20 @@ BeesHashTable::BeesHashTable(shared_ptr<BeesContext> ctx, string filename, off_t
 	THROW_CHECK2(runtime_error, m_void_ptr, m_bucket_ptr, m_void_ptr == m_bucket_ptr);
 	THROW_CHECK2(runtime_error, m_void_ptr, m_extent_ptr, m_void_ptr == m_extent_ptr);
 
-	{
-		// It's OK if this fails (e.g. kernel not built with CONFIG_TRANSPARENT_HUGEPAGE)
-		// We don't fork any more so DONTFORK isn't really needed
-		BEESTOOLONG("madvise(MADV_HUGEPAGE | MADV_DONTFORK)");
-		if (madvise(m_byte_ptr, m_size, MADV_HUGEPAGE | MADV_DONTFORK)) {
-			BEESLOG("mostly harmless: madvise(MADV_HUGEPAGE | MADV_DONTFORK) failed: " << strerror(errno));
+	// Give all the madvise hints that the kernel understands
+	const struct madv_flag {
+		const char *name;
+		int value;
+	} madv_flags[] = {
+		{ .name = "MADV_HUGEPAGE", .value = MADV_HUGEPAGE },
+		{ .name = "MADV_DONTFORK", .value = MADV_DONTFORK },
+		{ .name = "MADV_DONTDUMP", .value = MADV_DONTDUMP },
+		{ .name = "", .value = 0 },
+	};
+	for (auto fp = madv_flags; fp->value; ++fp) {
+		BEESTOOLONG("madvise(" << fp->name << ")");
+		if (madvise(m_byte_ptr, m_size, fp->value)) {
+			BEESLOG("madvise(..., " << fp->name << "): " << strerror(errno) << " (ignored)");
 		}
 	}