Re: [RFC v3] Support volatile range for anon vma

John Stultz
2012-12-11T18:45:36Z

On 12/10/2012 06:34 PM, Minchan Kim wrote:
> This still is [RFC v3] because just passed my simple test
> with TCMalloc tweaking.
>
> I hope more inputs from user-space allocator people and test patch
> with their allocator because it might need design change of arena
> management design for getting real vaule.
>
> Changelog from v2
>
> * Removing madvise(addr, length, MADV_NOVOLATILE).
> * add vmstat about the number of discarded volatile pages
> * discard volatile pages without promotion in reclaim path
>
> This is based on v3.6.
>
> - What's the madvise(addr, length, MADV_VOLATILE)?
>
> It's a hint that user deliver to kernel so kernel can *discard*
> pages in a range anytime.
>
> - What happens if user access page(ie, virtual address) discarded
> by kernel?
>
> The user can see zero-fill-on-demand pages as if madvise(DONTNEED).
>
> - What happens if user access page(ie, virtual address) doesn't
> discarded by kernel?
>
> The user can see old data without page fault.
>
> - What's different with madvise(DONTNEED)?
>
> System call semantic
>
> DONTNEED makes sure user always can see zero-fill pages after
> he calls madvise while VOLATILE can see zero-fill pages or
> old data.
I still need to really read and understand the patch, but at a high
level I'm not sure how this works. So does the VOLATILE flag get cleared
on any access, even if the pages have not been discarded? What happens
if an application wants to store non-volatile data in an area that was
once marked volatile. If there was never memory pressure, it seems the
volatility would persist with no way of removing it.

Either way, I feel that with this revision, specifically dropping the
NOVOLATILE call and the SIGBUS optimization the Mozilla folks suggested,
your implementation has drifted quite far from the concept I'm pushing.
While I hope we can still align the underlying mm implementation, I
might ask that you use a different term for the semantics you propose,
so we don't add too much confusion to the discussion.

Maybe you could call it DONTNEED_DEFERRED or something?

In the meantime, I'll be reading your patch in detail and seeing how we
might be able to combine our differing approaches.

thanks
-john