Re: [RFC] mm: remove swapcache page early

[Hugh Dickins]

On Fri, 29 Mar 2013, Minchan Kim wrote:
> On Thu, Mar 28, 2013 at 11:19:12AM -0700, Dan Magenheimer wrote:
> > > From: Minchan Kim [mailto:minchan@xxxxxxxxxx]
> > > On Wed, Mar 27, 2013 at 03:24:00PM -0700, Dan Magenheimer wrote:
> > > > > From: Hugh Dickins [mailto:hughd@xxxxxxxxxx]
> > > > > 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.)

Thanks for correcting me on zram (in earlier mail of this thread), yes,
I was forgetting about the swap_slot_free_notify entry point which lets
that memory be freed.

> 
> 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!

Be careful, although Dan is right that something like this can be
done for zram, I believe you will find that it needs a little more:
either a separate new entry point (not my preference) or a flags arg
(or boolean) added to swap_slot_free_notify.

Because this is a different operation: end_swap_bio_read() wants
to free up zram's compressed copy of the page, but the swp_entry_t
must remain valid until swap_entry_free() can clear up the rest.
Precisely how much of the work each should do, you will discover.

Hugh

> 
> > 
> > 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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