linux-kernel - Re: [v4][PATCH 1/6] mm: swap: defer clearing of page

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Date:	Tue, 4 Jun 2013 13:41:39 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Dave Hansen <dave@...1.net>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, mgorman@...e.de,
	tim.c.chen@...ux.intel.com
Subject: Re: [v4][PATCH 1/6] mm: swap: defer clearing of page_private() for
 swap cache pages

On Mon, Jun 03, 2013 at 07:53:01AM -0700, Dave Hansen wrote:
> On 06/02/2013 10:40 PM, Minchan Kim wrote:
> >> > diff -puN mm/vmscan.c~__delete_from_swap_cache-dont-clear-page-private mm/vmscan.c
> >> > --- linux.git/mm/vmscan.c~__delete_from_swap_cache-dont-clear-page-private	2013-05-30 16:07:50.632079492 -0700
> >> > +++ linux.git-davehans/mm/vmscan.c	2013-05-30 16:07:50.637079712 -0700
> >> > @@ -494,6 +494,8 @@ static int __remove_mapping(struct addre
> >> >  		__delete_from_swap_cache(page);
> >> >  		spin_unlock_irq(&mapping->tree_lock);
> >> >  		swapcache_free(swap, page);
> >> > +		set_page_private(page, 0);
> >> > +		ClearPageSwapCache(page);
> > It it worth to support non-atomic version of ClearPageSwapCache?
> 
> Just for this, probably not.
> 
> It does look like a site where it would be theoretically safe to use
> non-atomic flag operations since the page is on a one-way trip to the
> allocator at this point and the __clear_page_locked() now happens _just_
> after this code.

True.

> 
> But, personally, I'm happy to leave it as-is.  The atomic vs. non-atomic
> flags look to me like a micro-optimization that we should use when we
> _know_ there will be some tangible benefit.  Otherwise, they're just
> something extra for developers to trip over and cause very subtle bugs.

I just asked it because when I read the description of patchset, I felt
you were very sensitive to the atomic operation on many CPU system with
several sockets. Anyway, if you don't want it, I don't mind it, either.

-- 
Kind regards,
Minchan Kim
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