Re: [PATCH 01/02] swap: discard while swapping only ifSWAP_FLAG_DISCARD_PAGES

[Rafael Aquini]

On Sun, May 26, 2013 at 10:55:32AM -0400, KOSAKI Motohiro wrote:
> On Sun, May 26, 2013 at 9:52 AM, Rafael Aquini <aquini@xxxxxxxxxx> wrote:
> > On Sun, May 26, 2013 at 07:44:56AM -0400, KOSAKI Motohiro wrote:
> >> > +                       /*
> >> > +                        * By flagging sys_swapon, a sysadmin can tell us to
> >> > +                        * either do sinle-time area discards only, or to just
> >> > +                        * perform discards for released swap page-clusters.
> >> > +                        * Now it's time to adjust the p->flags accordingly.
> >> > +                        */
> >> > +                       if (swap_flags & SWAP_FLAG_DISCARD_ONCE)
> >> > +                               p->flags &= ~SWP_PAGE_DISCARD;
> >> > +                       else if (swap_flags & SWAP_FLAG_DISCARD_PAGES)
> >> > +                               p->flags &= ~SWP_AREA_DISCARD;
> >>
> >> When using old swapon(8), this code turn off both flags, right
> >
>  > As the flag that enables swap discards SWAP_FLAG_DISCARD remains meaning the
> > same it meant before, when using old swapon(8) (SWP_PAGE_DISCARD|SWP_AREA_DISCARD)
> 
> But old swapon(8) don't use neigher SWAP_FLAG_DISCARD_ONCE nor
> SWAP_FLAG_DISCARD_PAGES.  It uses only SWAP_FLAG_DISCARD. So, this
> condition disables both SWP_PAGE_DISCARD and SWP_AREA_DISCARD.
>

This condition _only_ disables one of the new flags orthogonally if swapon(8)
flags a policy to sys_swapon. As old swapon(8) can only flag SWAP_FLAG_DISCARD,
the original behavior is kept. Nothing will change when one is using an old
swapon(8) with this changeset. 

 
> And you changed that SWP_DISCARDABLE is not checked in IO path  at all.
> 
> >-               if (si->flags & SWP_DISCARDABLE) {
> >+               if (si->flags & SWP_PAGE_DISCARD) {
> 

And this is exactly what this change is about -- only enabling that particular
I/O path if we've been told to discard swap page-clusters. Notice that having
SWP_PAGE_DISCARD flagged already implies SWP_DISCARDABLE.


> I suggest new swapon(8) don't pass SWP_DISCARDABLE and kernel handle
> SWP_DISCARDABLE as (SWAP_FLAG_DISCARD_ONCE | SWAP_FLAG_DISCARD_PAGES).

As the old swapon(8) case can only pass SWAP_FLAG_DISCARD along, this change
would nullify the backwards compatibility, wouldn't it?



> 
> Optionally, warn SWP_DISCARDABLE is a good idea.
> 
> 
> > will remain flagged when discard is enabled, so we keep doing discards the same way
> > we did before (at swapon, and for every released page-cluster).
> > The flags are removed orthogonally only when the new swapon(8) selects one of the
> > particular discard policy available by using either SWAP_FLAG_DISCARD_ONCE,
> > or SWAP_FLAG_DISCARD_PAGES flags.
> >
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