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

[KOSAKI Motohiro]

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.

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) {

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

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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