Messages in this thread | | | Date | Sun, 26 May 2013 12:30:17 -0300 | From | Rafael Aquini <> | Subject | Re: [PATCH 01/02] swap: discard while swapping only if SWAP_FLAG_DISCARD_PAGES |
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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@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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