[PATCH 11/14] mm: use SWP_SYNCHRONOUS_IO more intelligently

[Minchan Kim]

Thanks for Ccing me, Shakeel.

On Mon, Jul 20, 2020 at 10:52:55AM -0700, Shakeel Butt wrote:
> +Minchan Kim
> 
> On Mon, Jul 20, 2020 at 12:52 AM Christoph Hellwig <hch at lst.de> wrote:
> >
> > There is no point in trying to call bdev_read_page if SWP_SYNCHRONOUS_IO
> > is not set, as the device won't support it.  Also there is no point in
> > trying a bio submission if bdev_read_page failed.
> 
> This will at least break the failure path of zram_rw_page().

Yes, it needs post processing for error propagaion like *page* handling
part in end_swap_bio_read(mostly, PG_error and PG_uptodate with pr_alert).
bdev_read_page's sematic doesn't need to be synchronous so it could just
submit the IO request and complete the IO afterward. In that case, we
need right error handling, too if the IO encoutered error. BIO fallback
makes it simple.

 * bdev_read_page() - Start reading a page from a block device
 * @bdev: The device to read the page from
 * @sector: The offset on the device to read the page to (need not be aligned)
 * @page: The page to read
 *
 * On entry, the page should be locked.  It will be unlocked when the page
 * has been read.  If the block driver implements rw_page synchronously,
 * that will be true on exit from this function, but it need not be.
 *
 * Errors returned by this function are usually "soft", eg out of memory, or
 * queue full; callers should try a different route to read this page rather
 * than propagate an error back up the stack.

The other concern about this patch is zram have used rw_page for a long
time even though sometime it doesn't declare BDI_CAP_SYNCHRONOUS_IO by itself
because rw_page shows 4~5% bandwidth improvement compared to bio-based.
The performance gain becomes more important these day because compressor
becomes more fast day by day.

> 
> >
> > Signed-off-by: Christoph Hellwig <hch at lst.de>
> > ---
> >  mm/page_io.c | 7 +++++--
> >  1 file changed, 5 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/page_io.c b/mm/page_io.c
> > index ccda7679008851..63b44b8221af0f 100644
> > --- a/mm/page_io.c
> > +++ b/mm/page_io.c
> > @@ -403,8 +403,11 @@ int swap_readpage(struct page *page, bool synchronous)
> >                 goto out;
> >         }
> >
> > -       ret = bdev_read_page(sis->bdev, swap_page_sector(page), page);
> > -       if (!ret) {
> > +       if (sis->flags & SWP_SYNCHRONOUS_IO) {
> > +               ret = bdev_read_page(sis->bdev, swap_page_sector(page), page);
> > +               if (ret)
> > +                       goto out;
> > +
> >                 if (trylock_page(page)) {
> >                         swap_slot_free_notify(page);
> >                         unlock_page(page);
> > --
> > 2.27.0
> >