[PATCH v14 05/21] fsverity: improve flushing performance of fsverity_fill_zerohash

The current version calls flush_dcache_folio(), in memcpy_to_folio(), to
flush whole folio on every digest (which is 128 for 4k) on the HIGHMEM
systems. Open code folio mapping and flushing to copy all digests at
once.

Reported-by: Eric Biggers <[email protected]>
Link: https://lore.kernel.org/linux-fsdevel/20260401222717.GH2466@quark/
Signed-off-by: Andrey Albershteyn <[email protected]>
---
 fs/verity/pagecache.c | 28 ++++++++++++++++++++++++++--
 1 file changed, 26 insertions(+), 2 deletions(-)

diff --git a/fs/verity/pagecache.c b/fs/verity/pagecache.c
index 9d82e6b74ba1..911207dc0ef7 100644
--- a/fs/verity/pagecache.c
+++ b/fs/verity/pagecache.c
@@ -68,14 +68,38 @@ EXPORT_SYMBOL_GPL(generic_readahead_merkle_tree);
 void fsverity_fill_zerohash(struct folio *folio, size_t offset, size_t len,
 			      struct fsverity_info *vi)
 {
-	size_t off = offset;
+	size_t off;
 
 	WARN_ON_ONCE(!IS_ALIGNED(offset, vi->tree_params.digest_size));
 	WARN_ON_ONCE(!IS_ALIGNED(len, vi->tree_params.digest_size));
+#ifdef CONFIG_HIGHMEM
+	WARN_ON_ONCE(offset + len > folio_size(folio));
 
-	for (; off < (offset + len); off += vi->tree_params.digest_size)
+	do {
+		void *vaddr = kmap_local_folio(folio, offset);
+		void *to = vaddr;
+
+		off = len;
+
+		if (folio_test_partial_kmap(folio) &&
+		    off > PAGE_SIZE - offset_in_page(offset))
+			off = PAGE_SIZE - offset_in_page(offset);
+		for (; to < (vaddr + off); to += vi->tree_params.digest_size)
+			memcpy(to, vi->tree_params.zero_digest,
+				vi->tree_params.digest_size);
+		kunmap_local(vaddr);
+
+		offset += off;
+		len -= off;
+	} while (len > 0);
+
+	flush_dcache_folio(folio);
+#else
+	for (off = offset; off < (offset + len);
+			off += vi->tree_params.digest_size)
 		memcpy_to_folio(folio, off, vi->tree_params.zero_digest,
 				vi->tree_params.digest_size);
+#endif
 }
 EXPORT_SYMBOL_GPL(fsverity_fill_zerohash);
 
-- 
2.54.0