doc: Explain how the UFFD handler gets information about pagesize

Document the new part of the initial handshake where Firecracker tells
the UFFD handler about page size in KiB.

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

Author: roypat

CHANGELOG.md

@@ -31,6 +31,10 @@ and this project adheres to
   supported snapshot version format. This change renders all previous
   Firecracker snapshots (up to Firecracker version v1.6.0) incompatible with the
   current Firecracker version.
+- [#4449](https://github.com/firecracker-microvm/firecracker/pull/4449): Added
+  information about page size to the payload Firecracker sends to the UFFD
+  handler. Each memory region object now contains a `page_size_kib` field. See
+  also the [hugepages documentation](docs/hugepages.md).
 
 ### Fixed
 

docs/hugepages.md

@@ -33,6 +33,11 @@ microVMs backed with huge pages can only be restored via UFFD. Lastly, note that
 even for guests backed by huge pages, differential snapshots will always track
 write accesses to guest memory at 4K granularity.
 
+When restoring snapshots via UFFD, Firecracker will send the configured page
+size (in KiB) for each memory region as part of the initial handshake, as
+described in our documentation on
+[UFFD-assisted snapshot-restore](snapshotting/handling-page-faults-on-snapshot-resume.md).
+
 ## Known Limitations
 
 Currently, hugetlbfs support is mutually exclusive with the following

docs/snapshotting/handling-page-faults-on-snapshot-resume.md

@@ -91,7 +91,8 @@ Firecracker and the page fault handler.
 ![](../images/uffd_flow3.png)
 
 - Firecracker passes the userfault file descriptor and the guest memory layout
-  to the page fault handler process through the socket.
+  (e.g. dimensions of each memory region, and their [page size](../hugepages.md)
+  in KiB) to the page fault handler process through the socket.
 
 ![](../images/uffd_flow4.png)
 
@@ -106,7 +107,7 @@ Firecracker and the page fault handler.
   happens, the page fault handler issues `UFFDIO_COPY` to load the previously
   mmaped file contents into the correspondent memory region.
 
-After Firecracker sends the payload (i.e mem mappings and file descriptor), no
+After Firecracker sends the payload (i.e. mem mappings and file descriptor), no
 other communication happens on the UDS socket (or otherwise) between Firecracker
 and the page fault handler process.