linux-kernel - Re: [PATCH v2] mm: show proportional swap share of the mapping

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Date:	Thu, 16 Jul 2015 08:49:55 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Hugh Dickins <hughd@...gle.com>,
	Bongkyu Kim <bongkyu.kim@....com>,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH v2] mm: show proportional swap share of the mapping

On Tue, Jul 14, 2015 at 02:07:08PM -0700, Andrew Morton wrote:
> On Mon, 15 Jun 2015 22:06:54 +0900 Minchan Kim <minchan@...nel.org> wrote:
> 
> > We want to know per-process workingset size for smart memory management
> > on userland and we use swap(ex, zram) heavily to maximize memory efficiency
> > so workingset includes swap as well as RSS.
> > 
> > On such system, if there are lots of shared anonymous pages, it's
> > really hard to figure out exactly how many each process consumes
> > memory(ie, rss + wap) if the system has lots of shared anonymous
> > memory(e.g, android).
> > 
> > This patch introduces SwapPss field on /proc/<pid>/smaps so we can get
> > more exact workingset size per process.
> > 
> > ...
> >
> > +int swp_swapcount(swp_entry_t entry)
> > +{
> > +	int count, tmp_count, n;
> > +	struct swap_info_struct *p;
> > +	struct page *page;
> > +	pgoff_t offset;
> > +	unsigned char *map;
> > +
> > +	p = swap_info_get(entry);
> > +	if (!p)
> > +		return 0;
> > +
> > +	count = swap_count(p->swap_map[swp_offset(entry)]);
> > +	if (!(count & COUNT_CONTINUED))
> > +		goto out;
> > +
> > +	count &= ~COUNT_CONTINUED;
> > +	n = SWAP_MAP_MAX + 1;
> > +
> > +	offset = swp_offset(entry);
> > +	page = vmalloc_to_page(p->swap_map + offset);
> > +	offset &= ~PAGE_MASK;
> > +	VM_BUG_ON(page_private(page) != SWP_CONTINUED);
> > +
> > +	do {
> > +		page = list_entry(page->lru.next, struct page, lru);
> > +		map = kmap_atomic(page) + offset;
> > +		tmp_count = *map;
> > +		kunmap_atomic(map);
> 
> A little thing: I've never liked the way that kunmap_atomic() accepts
> any address within the page.  It's weird, and it makes the reviewer
> have to scramble around to make sure the offset can never be >=
> PAGE_SIZE.

Very ture. I was bitten by that.
Thanks for the clean up.

> 
> We can easily avoid doing it here:
> 
> --- a/mm/swapfile.c~mm-show-proportional-swap-share-of-the-mapping-fix
> +++ a/mm/swapfile.c
> @@ -904,8 +904,8 @@ int swp_swapcount(swp_entry_t entry)
>  
>  	do {
>  		page = list_entry(page->lru.next, struct page, lru);
> -		map = kmap_atomic(page) + offset;
> -		tmp_count = *map;
> +		map = kmap_atomic(page);
> +		tmp_count = map[offset];
>  		kunmap_atomic(map);
>  
>  		count += (tmp_count & ~COUNT_CONTINUED) * n;
> 
> > +		count += (tmp_count & ~COUNT_CONTINUED) * n;
> > +		n *= (SWAP_CONT_MAX + 1);
> > +	} while (tmp_count & COUNT_CONTINUED);
> > +out:
> > +	spin_unlock(&p->lock);
> > +	return count;
> > +}
> >

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