iomap: Copy larger chunks from userspace

If we have a large folio, we can copy in larger chunks than PAGE_SIZE.
Start at the maximum page cache size and shrink by half every time we
hit the "we are short on memory" problem.
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

fs/iomap/buffered-io.c

@@ -769,6 +769,7 @@ static size_t iomap_write_end(struct iomap_iter *iter, loff_t pos, size_t len,
 static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
 {
 	loff_t length = iomap_length(iter);
+	size_t chunk = PAGE_SIZE << MAX_PAGECACHE_ORDER;
 	loff_t pos = iter->pos;
 	ssize_t written = 0;
 	long status = 0;
@@ -777,15 +778,12 @@ static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
 
 	do {
 		struct folio *folio;
-		struct page *page;
-		unsigned long offset;	/* Offset into pagecache page */
-		unsigned long bytes;	/* Bytes to write to page */
+		size_t offset;		/* Offset into folio */
+		size_t bytes;		/* Bytes to write to folio */
 		size_t copied;		/* Bytes copied from user */
 
-		offset = offset_in_page(pos);
-		bytes = min_t(unsigned long, PAGE_SIZE - offset,
-						iov_iter_count(i));
-again:
+		offset = pos & (chunk - 1);
+		bytes = min(chunk - offset, iov_iter_count(i));
 		status = balance_dirty_pages_ratelimited_flags(mapping,
 							       bdp_flags);
 		if (unlikely(status))
@@ -815,12 +813,14 @@ static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
 		if (iter->iomap.flags & IOMAP_F_STALE)
 			break;
 
-		page = folio_file_page(folio, pos >> PAGE_SHIFT);
-		if (mapping_writably_mapped(mapping))
-			flush_dcache_page(page);
+		offset = offset_in_folio(folio, pos);
+		if (bytes > folio_size(folio) - offset)
+			bytes = folio_size(folio) - offset;
 
-		copied = copy_page_from_iter_atomic(page, offset, bytes, i);
+		if (mapping_writably_mapped(mapping))
+			flush_dcache_folio(folio);
 
+		copied = copy_folio_from_iter_atomic(folio, offset, bytes, i);
 		status = iomap_write_end(iter, pos, bytes, copied, folio);
 
 		if (unlikely(copied != status))
@@ -836,11 +836,13 @@ static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
 			 */
 			if (copied)
 				bytes = copied;
-			goto again;
-		}
+			if (chunk > PAGE_SIZE)
+				chunk /= 2;
+		} else {
 			pos += status;
 			written += status;
 			length -= status;
+		}
 	} while (iov_iter_count(i) && length);
 
 	if (status == -EAGAIN) {