Re: [RFC v2] Support volatile range for anon vma

[Minchan Kim]

Hi KOSAKI,

On Wed, Oct 31, 2012 at 06:56:05PM -0400, KOSAKI Motohiro wrote:
> >> > Allocator should call madvise(MADV_NOVOLATILE) before reusing for
> >> > allocating that area to user. Otherwise, accessing of volatile range
> >> > will meet SIGBUS error.
> >>
> >> Well, why?  It would be easy enough for the fault handler to give
> >> userspace a new, zeroed page at that address.
> >
> > Note: MADV_DONTNEED already has this (nice) property.
> 
> I don't think I strictly understand this patch. but maybe I can answer why
> userland and malloc folks don't like MADV_DONTNEED.
> 
> glibc malloc discard freed memory by using MADV_DONTNEED
> as tcmalloc. and it is often a source of large performance decrease.
> because of MADV_DONTNEED discard memory immediately and
> right after malloc() call fall into page fault and pagesize memset() path.
> then, using DONTNEED increased zero fill and cache miss rate.
> 
> At called free() time, malloc don't have a knowledge when next big malloc()
> is called. then, immediate discarding may or may not get good performance
> gain. (Ah, ok, the rate is not 5:5. then usually it is worth. but not everytime)
> 
> 
> In past, several developers tryied to avoid such situation, likes
> 
> - making zero page daemon and avoid pagesize zero fill at page fault
> - making new vma or page flags and mark as discardable w/o swap and
>   vmscan treat it. (like this and/or MADV_FREE)

Thanks for the information.
I realized by you I'm not first people to think of this idea.
Rik already tried it(https://lkml.org/lkml/2007/4/17/53) by new page flag
and even other OSes already have such good feature. And John's concept was
already tried long time ago (https://lkml.org/lkml/2005/11/1/384)

Hmm, I look over Rik's thread but couldn't find why it wasn't merged
at that time. Anyone know it?

> - making new process option and avoid page zero fill from page fault path.
>   (yes, it is big incompatibility and insecure. but some embedded folks thought
>    they are acceptable downside)
> - etc
> 
> 
> btw, I'm not sure this patch is better for malloc because current MADV_DONTNEED
> don't need mmap_sem and works very effectively when a lot of threads case.
> taking mmap_sem might bring worse performance than DONTNEED. dunno.

It's a good point. 

Quote from my reply to Paul
"
In such case, we can change semantic so malloc doesn't need to call
madivse(NOVOLATILE) before allocating. Then, page fault handler have to
check whether this page fault happen by access of volatile vma. If so,
it could return zero page instead of SIGBUS and mark the vma isn't volatile
any more.
"
> 
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-- 
Kind regards,
Minchan Kim
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