mm: add folio_fill_tail() and use it in iomap

The iomap code was limited to PAGE_SIZE bytes; generalise it to cover
an arbitrary-sized folio, and move it to be a common helper.

[akpm@linux-foundation.org: fix folio_fill_tail(), per Andreas Gruenbacher]
Link: https://lkml.kernel.org/r/20231107212643.3490372-3-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Andreas Gruenbacher <agruenba@redhat.com>
Cc: Andreas Dilger <adilger.kernel@dilger.ca>
Cc: Darrick J. Wong <djwong@kernel.org>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/iomap/buffered-io.c

@@ -305,28 +305,18 @@ static int iomap_read_inline_data(const struct iomap_iter *iter,
 {
 	const struct iomap *iomap = iomap_iter_srcmap(iter);
 	size_t size = i_size_read(iter->inode) - iomap->offset;
-	size_t poff = offset_in_page(iomap->offset);
 	size_t offset = offset_in_folio(folio, iomap->offset);
-	void *addr;
 
 	if (folio_test_uptodate(folio))
 		return 0;
 
-	if (WARN_ON_ONCE(size > PAGE_SIZE - poff))
-		return -EIO;
-	if (WARN_ON_ONCE(size > PAGE_SIZE -
-			 offset_in_page(iomap->inline_data)))
-		return -EIO;
 	if (WARN_ON_ONCE(size > iomap->length))
 		return -EIO;
 	if (offset > 0)
 		ifs_alloc(iter->inode, folio, iter->flags);
 
-	addr = kmap_local_folio(folio, offset);
-	memcpy(addr, iomap->inline_data, size);
-	memset(addr + size, 0, PAGE_SIZE - poff - size);
-	kunmap_local(addr);
-	iomap_set_range_uptodate(folio, offset, PAGE_SIZE - poff);
+	folio_fill_tail(folio, offset, iomap->inline_data, size);
+	iomap_set_range_uptodate(folio, offset, folio_size(folio) - offset);
 	return 0;
 }
 

include/linux/highmem.h

@@ -521,6 +521,44 @@ static inline __must_check void *folio_zero_tail(struct folio *folio,
 	return kaddr;
 }
 
+/**
+ * folio_fill_tail - Copy some data to a folio and pad with zeroes.
+ * @folio: The destination folio.
+ * @offset: The offset into @folio at which to start copying.
+ * @from: The data to copy.
+ * @len: How many bytes of data to copy.
+ *
+ * This function is most useful for filesystems which support inline data.
+ * When they want to copy data from the inode into the page cache, this
+ * function does everything for them.  It supports large folios even on
+ * HIGHMEM configurations.
+ */
+static inline void folio_fill_tail(struct folio *folio, size_t offset,
+		const char *from, size_t len)
+{
+	char *to = kmap_local_folio(folio, offset);
+
+	VM_BUG_ON(offset + len > folio_size(folio));
+
+	if (folio_test_highmem(folio)) {
+		size_t max = PAGE_SIZE - offset_in_page(offset);
+
+		while (len > max) {
+			memcpy(to, from, max);
+			kunmap_local(to);
+			len -= max;
+			from += max;
+			offset += max;
+			max = PAGE_SIZE;
+			to = kmap_local_folio(folio, offset);
+		}
+	}
+
+	memcpy(to, from, len);
+	to = folio_zero_tail(folio, offset + len, to + len);
+	kunmap_local(to);
+}
+
 /**
  * memcpy_from_file_folio - Copy some bytes from a file folio.
  * @to: The destination buffer.