Re: [PATCH V5 6/6] proc: show MADV_FREE pages info in smaps

[Shaohua Li <shli@fb.com>]

On Wed, Mar 01, 2017 at 02:36:24PM +0100, Michal Hocko wrote:
> On Fri 24-02-17 13:31:49, Shaohua Li wrote:
> > show MADV_FREE pages info of each vma in smaps. The interface is for
> > diganose or monitoring purpose, userspace could use it to understand
> > what happens in the application. Since userspace could dirty MADV_FREE
> > pages without notice from kernel, this interface is the only place we
> > can get accurate accounting info about MADV_FREE pages.
> 
> I have just got to test this patchset and noticed something that was a
> bit surprising
> 
> madvise(mmap(len), len, MADV_FREE)
> Size:             102400 kB
> Rss:              102400 kB
> Pss:              102400 kB
> Shared_Clean:          0 kB
> Shared_Dirty:          0 kB
> Private_Clean:    102400 kB
> Private_Dirty:         0 kB
> Referenced:            0 kB
> Anonymous:        102400 kB
> LazyFree:         102368 kB
> 
> It took me a some time to realize that LazyFree is not accurate because
> there are still pages on the per-cpu lru_lazyfree_pvecs. I believe this
> is an implementation detail which shouldn't be visible to the userspace.
> Should we simply drain the pagevec? A crude way would be to simply
> lru_add_drain_all after we are done with the given range. We can also
> make this lru_lazyfree_pvecs specific but I am not sure this is worth
> the additional code.

Minchan's original patch includes a drain of pvec. I discard it because I think
it's not worth the effort. There aren't too many memory in the per-cpu vecs.
Like what you said, I doubt this is noticeable to userspace.

Thanks,
Shaohua


> ---
> diff --git a/mm/madvise.c b/mm/madvise.c
> index dc5927c812d3..d2c318db16c9 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -474,7 +474,7 @@ static int madvise_free_single_vma(struct vm_area_struct *vma,
>  	madvise_free_page_range(&tlb, vma, start, end);
>  	mmu_notifier_invalidate_range_end(mm, start, end);
>  	tlb_finish_mmu(&tlb, start, end);
> -
> +	lru_add_drain_all();
>  	return 0;
>  }
>  
> -- 
> Michal Hocko
> SUSE Labs