Re: [RFC] mm: remove swapcache page early

Minchan Kim
2013-03-29T01:18:06Z

On Thu, Mar 28, 2013 at 11:19:12AM -0700, Dan Magenheimer wrote:
> > From: Minchan Kim [mailto:minchan@kernel.org]
> > Subject: Re: [RFC] mm: remove swapcache page early
> >
> > Hi Dan,
> >
> > On Wed, Mar 27, 2013 at 03:24:00PM -0700, Dan Magenheimer wrote:
> > > > From: Hugh Dickins [mailto:hughd@google.com]
> > > > Subject: Re: [RFC] mm: remove swapcache page early
> > > >
> > > > I believe the answer is for frontswap/zmem to invalidate the frontswap
> > > > copy of the page (to free up the compressed memory when possible) and
> > > > SetPageDirty on the PageUptodate PageSwapCache page when swapping in
> > > > (setting page dirty so nothing will later go to read it from the
> > > > unfreed location on backing swap disk, which was never written).
> > >
> > > There are two duplication issues: (1) When can the page be removed
> > > from the swap cache after a call to frontswap_store; and (2) When
> > > can the page be removed from the frontswap storage after it
> > > has been brought back into memory via frontswap_load.
> > >
> > > This patch from Minchan addresses (1). The issue you are raising
> >
> > No. I am addressing (2).
> >
> > > here is (2). You may not know that (2) has recently been solved
> > > in frontswap, at least for zcache. See frontswap_exclusive_gets_enabled.
> > > If this is enabled (and it is for zcache but not yet for zswap),
> > > what you suggest (SetPageDirty) is what happens.
> >
> > I am blind on zcache so I didn't see it. Anyway, I'd like to address it
> > on zram and zswap.
>
> Zswap can enable it trivially by adding a function call in init_zswap.
> (Note that it is not enabled by default for all frontswap backends
> because it is another complicated tradeoff of cpu time vs memory space
> that needs more study on a broad set of workloads.)
>
> I wonder if something like this would have a similar result for zram?
> (Completely untested... snippet stolen from swap_entry_free with
> SetPageDirty added... doesn't compile yet, but should give you the idea.)

Nice idea!

After I see your patch, I realized it was Hugh's suggestion and
you implemented it in proper place.

Will resend it after testing. Maybe nextweek.
Thanks!

>
> diff --git a/mm/page_io.c b/mm/page_io.c
> index 56276fe..2d10988 100644
> --- a/mm/page_io.c
> +++ b/mm/page_io.c
> @@ -81,7 +81,17 @@ void end_swap_bio_read(struct bio *bio, int err)
> iminor(bio->bi_bdev->bd_inode),
> (unsigned long long)bio->bi_sector);
> } else {
> + struct swap_info_struct *sis;
> +
> SetPageUptodate(page);
> + sis = page_swap_info(page);
> + if (sis->flags & SWP_BLKDEV) {
> + struct gendisk *disk = sis->bdev->bd_disk;
> + if (disk->fops->swap_slot_free_notify) {
> + SetPageDirty(page);
> + disk->fops->swap_slot_free_notify(sis->bdev,
> + offset);
> + }
> + }
> }
> unlock_page(page);
> bio_put(bio);
>
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--
Kind regards,
Minchan Kim