Christoph Hellwig <hch@xxxxxx> wrote: > On Thu, Aug 12, 2021 at 12:57:58PM +0100, David Howells wrote: > > 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. > > + 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
