btrfs: reduce div64 calls by limiting the number of stripes of a chunk to u32

There are quite some div64 calls inside btrfs_map_block() and its
variants.

Such calls are for @stripe_nr, where @stripe_nr is the number of
stripes before our logical bytenr inside a chunk.

However we can eliminate such div64 calls by just reducing the width of
@stripe_nr from 64 to 32.

This can be done because our chunk size limit is already 10G, with fixed
stripe length 64K.
Thus a U32 is definitely enough to contain the number of stripes.

With such width reduction, we can get rid of slower div64, and extra
warning for certain 32bit arch.

This patch would do:

- Add a new tree-checker chunk validation on chunk length
  Make sure no chunk can reach 256G, which can also act as a bitflip
  checker.

- Reduce the width from u64 to u32 for @stripe_nr variables

- Replace unnecessary div64 calls with regular modulo and division
  32bit division and modulo are much faster than 64bit operations, and
  we are finally free of the div64 fear at least in those involved
  functions.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/fs/btrfs/tree-checker.c b/fs/btrfs/tree-checker.c
index baad1ed7e111..e2b54793bf0c 100644
--- a/fs/btrfs/tree-checker.c
+++ b/fs/btrfs/tree-checker.c
@@ -849,6 +849,20 @@ int btrfs_check_chunk_valid(struct extent_buffer *leaf,
 			  stripe_len);
 		return -EUCLEAN;
 	}
+	/*
+	 * We artificially limit the chunk size, so that the number of stripes
+	 * inside a chunk can be fit into a U32.  The current limit (256G) is
+	 * way too large for real world usage anyway, and it's also much larger
+	 * than our existing limit (10G).
+	 *
+	 * Thus it should be a good way to catch obvious bitflips.
+	 */
+	if (unlikely(length >= ((u64)U32_MAX << BTRFS_STRIPE_LEN_SHIFT))) {
+		chunk_err(leaf, chunk, logical,
+			  "chunk length too large: have %llu limit %llu",
+			  length, (u64)U32_MAX << BTRFS_STRIPE_LEN_SHIFT);
+		return -EUCLEAN;
+	}
 	if (unlikely(type & ~(BTRFS_BLOCK_GROUP_TYPE_MASK |
 			      BTRFS_BLOCK_GROUP_PROFILE_MASK))) {
 		chunk_err(leaf, chunk, logical,