Re: [PATCH -mm] vmscan: make mapped executable pages the first class citizen

Wu Fengguang
2009-05-10T10:04:20Z

On Sun, May 10, 2009 at 05:29:43PM +0800, KOSAKI Motohiro wrote:
> >> >> The patch seems reasonable but the changelog and the (non-existent)
> >> >> design documentation could do with a touch-up.
> >> >
> >> > Is it right that I as a user can do things like mmap my database
> >> > PROT_EXEC to get better database numbers by making other
> >> > stuff swap first ?
> >> >
> >> > You seem to be giving everyone a "nice my process up" hack.
> >>
> >> How about this?
> >
> > Why it deserves more tricks? PROT_EXEC pages are rare.
> > If user space is to abuse PROT_EXEC, let them be for it ;-)
>
> yes, typicall rare.
> tha problem is, user program _can_ use PROT_EXEC for get higher priority
> ahthough non-executable memory.

- abuses should be rare
- large scale abuses will be even more rare,
- the resulted vmscan overheads are the *expected* side effect
- the side effects are still safe

So if that's what they want, let them have it to their heart's content.

You know it's normal for many users/apps to care only about the result.
When they want something but cannot get it from the smarter version of
PROT_EXEC heuristics, they will go on to devise more complicated tricks.

In the end both sides loose.

If the abused case is important enough, then let's introduce a feature
to explicitly prioritize the pages. But let's leave the PROT_EXEC case
simple.

> In general, static priority mechanism have one weakness. if all object
> have higher priority, it break priority mechanism.

Yup.

> >> then, this patch don't change kernel reclaim policy.
> >>
> >> anyway, user process non-changable preventing "nice my process up
> >> hack" seems makes sense to me.
> >>
> >> test result:
> >>
> >> echo 100 > /proc/sys/vm/dirty_ratio
> >> echo 100 > /proc/sys/vm/dirty_background_ratio
> >> run modified qsbench (use mmap(PROT_EXEC) instead malloc)
> >>
> >>            active2active vs active2inactive ratio
> >> before    5:5
> >> after       1:9
> >
> > Do you have scripts for producing such numbers? I'm dreaming to have
> > such tools :-)
>
> I made stastics showing patch for testing, hehe :)

I see :)

Thanks,
Fengguang

> ---
> include/linux/vmstat.h | 1 +
> mm/vmstat.c | 1 +
> 2 files changed, 2 insertions(+)
>
> Index: b/include/linux/vmstat.h
> ===================================================================
> --- a/include/linux/vmstat.h 2009-02-17 07:34:38.000000000 +0900
> +++ b/include/linux/vmstat.h 2009-05-10 02:36:37.000000000 +0900
> @@ -51,6 +51,7 @@ enum vm_event_item { PGPGIN, PGPGOUT, PS
> UNEVICTABLE_PGSTRANDED, /* unable to isolate on unlock */
> UNEVICTABLE_MLOCKFREED,
> #endif
> + FOR_ALL_ZONES(PGA2A),
> NR_VM_EVENT_ITEMS
> };
>
> Index: b/mm/vmstat.c
> ===================================================================
> --- a/mm/vmstat.c 2009-05-10 01:08:36.000000000 +0900
> +++ b/mm/vmstat.c 2009-05-10 02:37:18.000000000 +0900
> @@ -708,6 +708,7 @@ static const char * const vmstat_text[]
> "unevictable_pgs_stranded",
> "unevictable_pgs_mlockfreed",
> #endif
> + TEXTS_FOR_ZONES("pga2a")
> #endif
> };
>
>
>
> >> please don't ask performance number. I haven't reproduce Wu's patch
> >> improvemnt ;)
> >
> > That's why I decided to "explain" instead of "benchmark" the benefits
> > of my patch, hehe.
>
> okey, I see.