From: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
To: Hugh Dickins <hughd@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Minchan Kim <minchan.kim@gmail.com>,
Rik van Riel <riel@redhat.com>, Shaohua Li <shaohua.li@intel.com>,
Eric Dumazet <eric.dumazet@gmail.com>,
Johannes Weiner <hannes@cmpxchg.org>,
Michel Lespinasse <walken@google.com>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH 2/2] SHM_UNLOCK: fix Unevictable pages stranded after swap
Date: Mon, 09 Jan 2012 18:09:32 -0500 [thread overview]
Message-ID: <4F0B73AC.7000504@gmail.com> (raw)
In-Reply-To: <alpine.LSU.2.00.1201091342300.1272@eggly.anvils>
(1/9/12 5:25 PM), Hugh Dickins wrote:
> On Mon, 9 Jan 2012, KOSAKI Motohiro wrote:
>> 2012/1/6 Hugh Dickins<hughd@google.com>:
>
> [ check_move_unevictable_page(s) ]
>
>>>
>>> Leave out the "rotate unevictable list" block: that's a leftover
>>> from when this was used for /proc/sys/vm/scan_unevictable_pages,
>>> whose flawed handling involved looking at pages at tail of LRU.
>>>
>>> Was there significance to the sequence first ClearPageUnevictable,
>>> then test page_evictable, then SetPageUnevictable here? I think
>>> not, we're under LRU lock, and have no barriers between those.
>>
>> If I understand correctly, this is not exactly correct. Because of,
>
> Thank you for giving it serious thought:
> such races are hard work to think about.
>
>> PG_mlocked operation is not protected by LRU lock. So, I think we
>
> Right. But I don't see that I've made a significant change there.
>
> I may be being lazy, and rushing back to answer you, without giving
> constructive thought to what the precise race is that you see, and
> how we might fix it. If the case you have in mind is easy for you
> to describe in detail, please do so; but don't hesitate to tell me
> to my own work for myself!
Bah! I was moron. I now think your code is right.
spin_lock(lru_lock)
if (page_evictable(page))
blah blah blah
spin_unlock(lru_lock)
is always safe. Counter part should have following code and
waiting spin_lock(lru_lock) in isolate_lru_page().
if (!isolate_lru_page(page))
putback_lru_page(page);
then, even if check_move_unevictable_pages() observed wrong page status,
putback_lru_page() should put back the page into right lru.
I'm very sorry for annoying you.
Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Probably, page_evictable() might be needed some additional comments. But
I have no idea what comment clearly explain this complex rule.....
next prev parent reply other threads:[~2012-01-09 23:09 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-01-06 21:10 [PATCH 1/2] SHM_UNLOCK: fix long unpreemptible section Hugh Dickins
2012-01-06 21:12 ` [PATCH 2/2] SHM_UNLOCK: fix Unevictable pages stranded after swap Hugh Dickins
2012-01-09 20:42 ` KOSAKI Motohiro
2012-01-09 22:25 ` Hugh Dickins
2012-01-09 23:09 ` KOSAKI Motohiro [this message]
2012-01-09 23:56 ` Hugh Dickins
2012-01-15 0:20 ` Hugh Dickins
2012-01-07 8:28 ` [PATCH 1/2] SHM_UNLOCK: fix long unpreemptible section KOSAKI Motohiro
2012-01-15 0:18 ` Hugh Dickins
2012-01-18 22:37 ` Andrew Morton
2012-01-18 23:26 ` Hugh Dickins
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