linux-kernel - Re: [PATCH] mm/mlock: fix BUG_ON unlocked page for nolinear VMAs

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Date:	Wed, 18 Dec 2013 13:43:16 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Wanpeng Li <liwanp@...ux.vnet.ibm.com>
Cc:	Vlastimil Babka <vbabka@...e.cz>,
	Sasha Levin <sasha.levin@...cle.com>,
	Michel Lespinasse <walken@...gle.com>,
	Bob Liu <bob.liu@...cle.com>, npiggin@...e.de,
	kosaki.motohiro@...fujitsu.com, riel@...hat.com,
	David Rientjes <rientjes@...gle.com>,
	Mel Gorman <mgorman@...e.de>, Minchan Kim <minchan@...nel.org>,
	Hugh Dickins <hughd@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/mlock: fix BUG_ON unlocked page for nolinear VMAs

On Wed, 18 Dec 2013 17:23:03 +0800 Wanpeng Li <liwanp@...ux.vnet.ibm.com> wrote:

> >>diff --git a/mm/rmap.c b/mm/rmap.c
> >>index 55c8b8d..1e24813 100644
> >>--- a/mm/rmap.c
> >>+++ b/mm/rmap.c
> >>@@ -1347,6 +1347,7 @@ static int try_to_unmap_cluster(unsigned long cursor, unsigned int *mapcount,
> >>  	unsigned long end;
> >>  	int ret = SWAP_AGAIN;
> >>  	int locked_vma = 0;
> >>+	int we_locked = 0;
> >>
> >>  	address = (vma->vm_start + cursor) & CLUSTER_MASK;
> >>  	end = address + CLUSTER_SIZE;
> >>@@ -1385,9 +1386,15 @@ static int try_to_unmap_cluster(unsigned long cursor, unsigned int *mapcount,
> >>  		BUG_ON(!page || PageAnon(page));
> >>
> >>  		if (locked_vma) {
> >>-			mlock_vma_page(page);   /* no-op if already mlocked */
> >>-			if (page == check_page)
> >>+			if (page != check_page) {
> >>+				we_locked = trylock_page(page);
> >
> >If it's not us who has the page already locked, but somebody else, he
> >might unlock it at this point and then the BUG_ON in mlock_vma_page()
> >will trigger again.

yes, this patch is pretty weak.

> Any better idea is appreciated. ;-)

Remove the BUG_ON() from mlock_vma_page()?  Why was it added?
isolate_lru_page() and putback_lru_page() and *might* require
the page be locked, but I don't immediately see issues?
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