Re: [PATCH 09/23] MM: submit multipage reads for SWP_FS_OPS swap-space

[Christoph Hellwig]

On Mon, Jan 24, 2022 at 02:48:32PM +1100, NeilBrown wrote:
> swap_readpage() is given one page at a time, but maybe 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 in a pointer-to-pointer when swap_readpage can
> store state between calls - much like the effect of blk_plug.  After
> calling swap_readpage() some number of times, the state will be passed
> to swap_read_unplug() which can submit the combined request.
> 
> Some caller currently call blk_finish_plug() *before* the final call to
> swap_readpage(), so the last page cannot be included.  This patch moves
> blk_finish_plug() to after the last call, and calls swap_read_unplug()
> there too.

Buildbot complais that we might need a forward declaration of
struct swap_iocb, but otherwise this looks good:

Reviewed-by: Christoph Hellwig <hch@xxxxxx>