Christoph Hellwig [off-list ref] wrote:

On Thu, Aug 12, 2021 at 12:57:58PM +0100, David Howells wrote:

quoted

Make swap_readpage(), when accessing a swap file (SWP_FS_OPS) use
the ->direct_IO() method on the filesystem rather then ->readpage().

->direct_IO is just a helper for ->read_iter and ->write_iter, so please
don't call it directly.  It actually is slowly on its way out, with at
at least all of the iomap implementations not using it, as well as various
other file systems.

[Note that __swap_writepage() uses ->direct_IO().]

Calling ->write_iter is probably a bad idea here.  Imagine that it goes
through, say, generic_file_write_iter(), then __generic_file_write_iter() and
then generic_file_direct_write().  It adds a number of delays into the system,
including:

	- Taking the inode lock
	- Removing file privs
	- Cranking mtime, ctime, file version
	  - Doing mnt_want_write
	  - Setting the inode dirty
	- Waiting on pages in the range that are being written 
	- Walking over the pagecache to invalidate the range
	- Redoing the invalidation (can't be skipped since page 0 is pinned)

that we might want to skip as they'll end up being done for every page swapped
out.

quoted

+	ki = kzalloc(sizeof(*ki), GFP_KERNEL);
+	if (!ki)
+		return -ENOMEM;

for the synchronous case we could avoid this allocation and just use
arguments on stack.

True.

David