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

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Message-ID: <51ACADCD.70904@sr71.net>
Date:	Mon, 03 Jun 2013 07:53:01 -0700
From:	Dave Hansen <dave@...1.net>
To:	Minchan Kim <minchan@...nel.org>
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 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.

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.
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