add a comment about page size vs bpf ring buffer sizing

As requested by Patrick on:
https://android-review.git.corp.google.com/c/platform/system/bpf/+/2628969
bpfLoader: automatically bump ringbuffer size up to page size

Test: N/A, comment only
Signed-off-by: Maciej Żenczykowski <maze@google.com>
(cherry picked from https://android-review.googlesource.com/q/commit:28f01bb027e5daf9793ed9a436bf735942cd423e)
Merged-In: Ibbce6a72e5e25bf18bb89bca2f00d332ef824f73
Change-Id: Ibbce6a72e5e25bf18bb89bca2f00d332ef824f73

1 files changed, 6 insertions, 0 deletions

diff --git a/libbpf_android/Loader.cpp b/libbpf_android/Loader.cpp
index 231fe39..83f6a88 100644
--- a/libbpf_android/Loader.cpp
+++ b/libbpf_android/Loader.cpp
@@ -707,6 +707,9 @@ static bool mapMatchesExpectations(const unique_fd& fd, const string& mapName,
     if (type == BPF_MAP_TYPE_DEVMAP || type == BPF_MAP_TYPE_DEVMAP_HASH)
         desired_map_flags |= BPF_F_RDONLY_PROG;
 
+    // The .h file enforces that this is a power of two, and page size will
+    // also always be a power of two, so this logic is actually enough to
+    // force it to be a multiple of the page size, as required by the kernel.
     unsigned int desired_max_entries = mapDef.max_entries;
     if (type == BPF_MAP_TYPE_RINGBUF) {
         if (desired_max_entries < page_size) desired_max_entries = page_size;
@@ -849,6 +852,9 @@ static int createMaps(const char* elfPath, ifstream& elfFile, vector<unique_fd>&
             type = BPF_MAP_TYPE_HASH;
         }
 
+        // The .h file enforces that this is a power of two, and page size will
+        // also always be a power of two, so this logic is actually enough to
+        // force it to be a multiple of the page size, as required by the kernel.
         unsigned int max_entries = md[i].max_entries;
         if (type == BPF_MAP_TYPE_RINGBUF) {
             if (max_entries < page_size) max_entries = page_size;