doc: include interaction of dirty page tracking with hugepages

Extend the limitation section of the huge pages docs to include a note
about how dirty page tracking negates the benefits of huge pages.

Also add a cross-reference to the diff snapshot docs.

Signed-off-by: Patrick Roy <[email protected]>

Author: roypat

docs/hugepages.md

@@ -40,6 +40,11 @@ Firecracker features:
 
 - Memory Ballooning via the [Balloon Device](./ballooning.md)
 
+Furthermore, enabling dirty page tracking for hugepage memory negates the
+performance benefits of using huge pages. This is because KVM will
+unconditionally establish guest page tables at 4K granularity if dirty page
+tracking is enabled, even if the host users huge mappings.
+
 ## FAQ
 
 ### Why does Firecracker not offer a transparent huge pages (THP) setting?

docs/snapshotting/snapshot-support.md

@@ -318,6 +318,9 @@ in the snapshot by instead considering all pages that were _accessed_ during the
 VM's lifetime. This potentially results in bigger memory files (although they
 are still sparse), but avoids the runtime overhead of dirty page logging.
 
+*Note*: Dirty page tracking negates most of the benefits of
+[huge pages](../hugepages.md#known-limitations).
+
 Without dirty page tracking enabled, Firecracker uses the
 [`mincore(2)`][man mincore] syscall to determine which pages to include in the
 snapshot. As such, this mode of snapshot taking will only work _if swap is