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

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Date:   Thu, 27 Jan 2022 09:04:27 +1100
From:   "NeilBrown" <neilb@...e.de>
To:     "Mark Hemment" <markhemm@...glemail.com>
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>, linux-nfs@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 09/23] MM: submit multipage reads for SWP_FS_OPS swap-space

On Tue, 25 Jan 2022, Mark Hemment wrote:
> On Mon, 24 Jan 2022 at 03:52, 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>
> > ---
> >  mm/madvise.c    |    8 +++-
> >  mm/memory.c     |    2 +
> >  mm/page_io.c    |  102 +++++++++++++++++++++++++++++++++++--------------------
> >  mm/swap.h       |   16 +++++++--
> >  mm/swap_state.c |   19 +++++++---
> >  5 files changed, 98 insertions(+), 49 deletions(-)
> >
> ...
> > diff --git a/mm/page_io.c b/mm/page_io.c
> > index 6e32ca35d9b6..bcf655d650c8 100644
> > --- a/mm/page_io.c
> > +++ b/mm/page_io.c
> > @@ -390,46 +391,60 @@ int __swap_writepage(struct page *page, struct writeback_control *wbc,
> >  static void sio_read_complete(struct kiocb *iocb, long ret)
> >  {
> >         struct swap_iocb *sio = container_of(iocb, struct swap_iocb, iocb);
> > -       struct page *page = sio->bvec.bv_page;
> > -
> > -       if (ret != 0 && ret != PAGE_SIZE) {
> > -               SetPageError(page);
> > -               ClearPageUptodate(page);
> > -               pr_alert_ratelimited("Read-error on swap-device\n");
> > -       } else {
> > -               SetPageUptodate(page);
> > -               count_vm_event(PSWPIN);
> > +       int p;
> > +
> > +       for (p = 0; p < sio->pages; p++) {
> > +               struct page *page = sio->bvec[p].bv_page;
> > +               if (ret != 0 && ret != PAGE_SIZE * sio->pages) {
> > +                       SetPageError(page);
> > +                       ClearPageUptodate(page);
> > +                       pr_alert_ratelimited("Read-error on swap-device\n");
> > +               } else {
> > +                       SetPageUptodate(page);
> > +                       count_vm_event(PSWPIN);
> > +               }
> > +               unlock_page(page);
> >         }
> > -       unlock_page(page);
> >         mempool_free(sio, sio_pool);
> >  }
> 
> Trivial: on success, could be single call to count_vm_events(PSWPIN,
> sio->pages).
> Similar comment for PSWPOUT in sio_write_complete()
> 
> >
> > -static int swap_readpage_fs(struct page *page)
> > +static void swap_readpage_fs(struct page *page,
> > +                            struct swap_iocb **plug)
> >  {
> >         struct swap_info_struct *sis = page_swap_info(page);
> > -       struct file *swap_file = sis->swap_file;
> > -       struct address_space *mapping = swap_file->f_mapping;
> > -       struct iov_iter from;
> > -       struct swap_iocb *sio;
> > +       struct swap_iocb *sio = NULL;
> >         loff_t pos = page_file_offset(page);
> > -       int ret;
> > -
> > -       sio = mempool_alloc(sio_pool, GFP_KERNEL);
> > -       init_sync_kiocb(&sio->iocb, swap_file);
> > -       sio->iocb.ki_pos = pos;
> > -       sio->iocb.ki_complete = sio_read_complete;
> > -       sio->bvec.bv_page = page;
> > -       sio->bvec.bv_len = PAGE_SIZE;
> > -       sio->bvec.bv_offset = 0;
> >
> > -       iov_iter_bvec(&from, READ, &sio->bvec, 1, PAGE_SIZE);
> > -       ret = mapping->a_ops->swap_rw(&sio->iocb, &from);
> > -       if (ret != -EIOCBQUEUED)
> > -               sio_read_complete(&sio->iocb, ret);
> > -       return ret;
> > +       if (*plug)
> > +               sio = *plug;
> 
> 'plug' can be NULL when called from do_swap_page();
>         if (plug && *plug)

Thanks for catching that!  I actually want it to be
   if (plug)
       sio = *plug;

which nicely balances the
   if (plug)
       *plug = sio;
at the end of the function.

Thanks,
NeilBrown