fsverity: simplify Merkle tree readahead size calculation

First, calculate max_ra_pages more efficiently by using the bio size.

Second, calculate the number of readahead pages from the hash page
index, instead of calculating it ahead of time using the data page
index.  This ends up being a bit simpler, especially since level 0 is
last in the tree, so we can just limit the readahead to the tree size.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Andrey Albershteyn <aalbersh@redhat.com>
Tested-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Link: https://lore.kernel.org/r/20221223203638.41293-3-ebiggers@kernel.org

fs/verity/fsverity_private.h

@@ -46,7 +46,7 @@ struct merkle_tree_params {
 	unsigned int log_arity;		/* log2(hashes_per_block) */
 	unsigned int num_levels;	/* number of levels in Merkle tree */
 	u64 tree_size;			/* Merkle tree size in bytes */
-	unsigned long level0_blocks;	/* number of blocks in tree level 0 */
+	unsigned long tree_pages;	/* Merkle tree size in pages */
 
 	/*
 	 * Starting block index for each tree level, ordered from leaf level (0)

fs/verity/open.c

@@ -7,6 +7,7 @@
 
 #include "fsverity_private.h"
 
+#include <linux/mm.h>
 #include <linux/slab.h>
 
 static struct kmem_cache *fsverity_info_cachep;
@@ -97,7 +98,6 @@ int fsverity_init_merkle_tree_params(struct merkle_tree_params *params,
 			 params->log_arity;
 		blocks_in_level[params->num_levels++] = blocks;
 	}
-	params->level0_blocks = blocks_in_level[0];
 
 	/* Compute the starting block of each level */
 	offset = 0;
@@ -118,6 +118,7 @@ int fsverity_init_merkle_tree_params(struct merkle_tree_params *params,
 	}
 
 	params->tree_size = offset << log_blocksize;
+	params->tree_pages = PAGE_ALIGN(params->tree_size) >> PAGE_SHIFT;
 	return 0;
 
 out_err:

fs/verity/verify.c

@@ -74,7 +74,7 @@ static inline int cmp_hashes(const struct fsverity_info *vi,
  */
 static bool verify_page(struct inode *inode, const struct fsverity_info *vi,
 			struct ahash_request *req, struct page *data_page,
-			unsigned long level0_ra_pages)
+			unsigned long max_ra_pages)
 {
 	const struct merkle_tree_params *params = &vi->tree_params;
 	const unsigned int hsize = params->digest_size;
@@ -103,7 +103,8 @@ static bool verify_page(struct inode *inode, const struct fsverity_info *vi,
 		hash_at_level(params, index, level, &hindex, &hoffset);
 
 		hpage = inode->i_sb->s_vop->read_merkle_tree_page(inode, hindex,
-				level == 0 ? level0_ra_pages : 0);
+				level == 0 ? min(max_ra_pages,
+					params->tree_pages - hindex) : 0);
 		if (IS_ERR(hpage)) {
 			err = PTR_ERR(hpage);
 			fsverity_err(inode,
@@ -199,14 +200,13 @@ void fsverity_verify_bio(struct bio *bio)
 {
 	struct inode *inode = bio_first_page_all(bio)->mapping->host;
 	const struct fsverity_info *vi = inode->i_verity_info;
-	const struct merkle_tree_params *params = &vi->tree_params;
 	struct ahash_request *req;
 	struct bio_vec *bv;
 	struct bvec_iter_all iter_all;
 	unsigned long max_ra_pages = 0;
 
 	/* This allocation never fails, since it's mempool-backed. */
-	req = fsverity_alloc_hash_request(params->hash_alg, GFP_NOFS);
+	req = fsverity_alloc_hash_request(vi->tree_params.hash_alg, GFP_NOFS);
 
 	if (bio->bi_opf & REQ_RAHEAD) {
 		/*
@@ -218,24 +218,17 @@ void fsverity_verify_bio(struct bio *bio)
 		 * This improves sequential read performance, as it greatly
 		 * reduces the number of I/O requests made to the Merkle tree.
 		 */
-		bio_for_each_segment_all(bv, bio, iter_all)
-			max_ra_pages++;
-		max_ra_pages /= 4;
+		max_ra_pages = bio->bi_iter.bi_size >> (PAGE_SHIFT + 2);
 	}
 
 	bio_for_each_segment_all(bv, bio, iter_all) {
-		struct page *page = bv->bv_page;
-		unsigned long level0_index = page->index >> params->log_arity;
-		unsigned long level0_ra_pages =
-			min(max_ra_pages, params->level0_blocks - level0_index);
-
-		if (!verify_page(inode, vi, req, page, level0_ra_pages)) {
+		if (!verify_page(inode, vi, req, bv->bv_page, max_ra_pages)) {
 			bio->bi_status = BLK_STS_IOERR;
 			break;
 		}
 	}
 
-	fsverity_free_hash_request(params->hash_alg, req);
+	fsverity_free_hash_request(vi->tree_params.hash_alg, req);
 }
 EXPORT_SYMBOL_GPL(fsverity_verify_bio);
 #endif /* CONFIG_BLOCK */