bees: misc comment updates

These have been accumulating in unpublished bees commits.  Squash them all
into one.

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

src/bees-context.cc

@@ -891,9 +891,10 @@ BeesContext::resolve_addr_uncached(BeesAddress addr)
 
 	auto rt_age = resolve_timer.age();
 
-	// Avoid performance bug
 	BeesResolveAddrResult rv;
 	rv.m_biors = log_ino.m_iors;
+
+	// Avoid performance bug
 	if (sys_usage_delta < BEES_TOXIC_SYS_DURATION) {
 		rv.m_is_toxic = false;
 	} else {

src/bees-resolve.cc

@@ -229,6 +229,8 @@ BeesResolver::chase_extent_ref(const BtrfsInodeOffsetRoot &bior, BeesBlockData &
 	// Search near the resolved address for a matching data block.
 	// ...even if it's not compressed, we should do this sanity
 	// check before considering the block as a duplicate candidate.
+	// FIXME:  this is mostly obsolete now and we shouldn't do it here.
+	// Don't bother fixing it because it will all go away with (extent, offset) reads.
 	auto new_bbd = adjust_offset(haystack_bbd, needle_bbd);
 	if (new_bbd.empty()) {
 		// matching offset search failed

src/bees-roots.cc

@@ -1004,10 +1004,7 @@ BeesCrawl::fetch_extents()
 	sk.min_type = sk.max_type = BTRFS_EXTENT_DATA_KEY;
 	sk.min_offset = old_state.m_offset;
 	sk.min_transid = old_state.m_min_transid;
-	// Don't set max_transid here.	We want to see old extents with
-	// new references, and max_transid filtering in the kernel locks
-	// the filesystem while slowing us down.
-	// sk.max_transid = old_state.m_max_transid;
+	// Don't set max_transid to m_max_transid here.	 See below.
 	sk.max_transid = numeric_limits<uint64_t>::max();
 	sk.nr_items = BEES_MAX_CRAWL_ITEMS;
 
@@ -1077,7 +1074,6 @@ BeesCrawl::fetch_extents()
 		if (gen < get_state_end().m_min_transid) {
 			BEESCOUNT(crawl_gen_low);
 			++count_low;
-			// We want (need?) to scan these anyway?
 			// The header generation refers to the transid
 			// of the metadata page holding the current ref.
 			// This includes anything else in that page that
@@ -1085,17 +1081,22 @@ BeesCrawl::fetch_extents()
 			// old it is.
 			// The file_extent_generation refers to the
 			// transid of the extent item's page, which is
-			// a different approximation of what we want.
-			// Combine both of these filters to minimize
-			// the number of times we unnecessarily re-read
-			// an extent.
+			// what we really want when we are slicing up
+			// the extent data by transid.
 			continue;
 		}
 		if (gen > get_state_end().m_max_transid) {
 			BEESCOUNT(crawl_gen_high);
 			++count_high;
-			// We have to filter these here because we can't
-			// do it in the kernel.
+			// We want to see old extents with references in
+			// new pages, which means we have to get extent
+			// refs from every page older than min_transid,
+			// not every page between min_transid and
+			// max_transid.  This means that we will get
+			// refs to new extent data that we don't want to
+			// process yet, because we'll process it again
+			// on the next crawl cycle.  We filter out refs
+			// to new extents here.
 			continue;
 		}
 

src/bees.h

@@ -50,7 +50,7 @@ const off_t BLOCK_SIZE_MAX_EXTENT = 128 * 1024 * 1024;
 const off_t BLOCK_MASK_CLONE = BLOCK_SIZE_CLONE - 1;
 const off_t BLOCK_MASK_SUMS = BLOCK_SIZE_SUMS - 1;
 
-// Maximum temporary file size
+// Maximum temporary file size (maximum extent size for temporary copy)
 const off_t BLOCK_SIZE_MAX_TEMP_FILE = 1024 * 1024 * 1024;
 
 // Bucket size for hash table (size of one hash bucket)
@@ -330,7 +330,6 @@ public:
 
 	// Blocks with no physical address (not yet allocated, hole, or "other").
 	// PREALLOC blocks have a physical address so they're not magic enough to be handled here.
-	// Compressed blocks have a physical address but it's two-dimensional.
 	enum MagicValue {
 		ZERO,		// BeesAddress uninitialized
 		DELALLOC,	// delayed allocation