Re: [PATCH 07/18] MM: submit multipage write for SWP_FS_OPS swap-space

On Thu, 16 Dec 2021 at 23:56, NeilBrown [off-list ref] wrote:

swap_writepage() is given one page at a time, but may be called repeatedly
in succession.
For block-device swapspace, the blk_plug functionality allows the
multiple pages to be combined together at lower layers.
That cannot be used for SWP_FS_OPS as blk_plug may not exist - it is
only active when CONFIG_BLOCK=y.  Consequently all swap reads over NFS
are single page reads.

With this patch we pass a pointer-to-pointer via the wbc.
swap_writepage can store state between calls - much like the pointer
passed explicitly to swap_readpage.  After calling swap_writepage() some
number of times, the state will be passed to swap_write_unplug() which
can submit the combined request.

Signed-off-by: NeilBrown <redacted>
---
 include/linux/writeback.h |    7 +++
 mm/page_io.c              |   98 ++++++++++++++++++++++++++++++---------------
 mm/swap.h                 |    1
 mm/vmscan.c               |    9 +++-
 4 files changed, 80 insertions(+), 35 deletions(-)

...

+void swap_write_unplug(struct swap_iocb *sio)
+{
+       struct iov_iter from;
+       struct address_space *mapping = sio->iocb.ki_filp->f_mapping;
+       int ret;
+
+       iov_iter_bvec(&from, WRITE, sio->bvec, sio->pages,
+                     PAGE_SIZE * sio->pages);
+       ret = mapping->a_ops->swap_rw(&sio->iocb, &from);
+       if (ret != -EIOCBQUEUED)
+               sio_write_complete(&sio->iocb, ret);
+}
+

As swap_write_unplug() is called from vmscan.c, need an 'no-op'
version (in "swap.h") for when !CONFIG_SWAP

Mark