linux-kernel - Re: [PATCH 09/23] MM: submit multipage reads for SWP_FS

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Date:   Tue, 8 Feb 2022 12:07:42 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     NeilBrown <neilb@...e.de>
Cc:     Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Chuck Lever <chuck.lever@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>,
        Christoph Hellwig <hch@...radead.org>,
        David Howells <dhowells@...hat.com>,
        "open list:NFS, SUNRPC, AND..." <linux-nfs@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 09/23] MM: submit multipage reads for SWP_FS_OPS swap-space

Hi Neil,

On Mon, Jan 24, 2022 at 5:49 PM NeilBrown <neilb@...e.de> 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.
>
> Signed-off-by: NeilBrown <neilb@...e.de>

Thanks for your patch!

> --- a/mm/swap.h
> +++ b/mm/swap.h

> @@ -53,7 +62,8 @@ static inline unsigned int page_swap_flags(struct page *page)
>         return page_swap_info(page)->flags;
>  }
>  #else /* CONFIG_SWAP */
> -static inline int swap_readpage(struct page *page, bool do_poll)
> +static inline int swap_readpage(struct page *page, bool do_poll,
> +                               struct swap_iocb **plug);

Bogus semicolon.

>  {
>         return 0;
>  }

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds