On Sun, May 26, 2013 at 10:55:32AM -0400, KOSAKI Motohiro wrote:
> On Sun, May 26, 2013 at 9:52 AM, Rafael Aquini <aqu...@redhat.com> 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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