Re: [PATCH] fix double unlock_page() in 2.6.26-rc5-mm3 kernel BUG at mm/filemap.c:575!

[Lee Schermerhorn <Lee.Schermerhorn@hp.com>]

On Tue, 2008-06-17 at 11:32 +0900, KAMEZAWA Hiroyuki wrote:
> On Fri, 13 Jun 2008 11:30:46 -0400
> Lee Schermerhorn <Lee.Schermerhorn@hp.com> wrote:
> 
> > 1)  modified putback_lru_page() to drop page lock only if both page_mapping()
> >     NULL and page_count() == 1 [rather than VM_BUG_ON(page_count(page) != 1].
> 
> I'm sorry that I cannot catch the whole changes..
> 
> I cannot convice that this implicit behavior won't cause lock-up in future, again.
> Even if there are enough comments...
> 
> Why the page should be locked when it is put back to LRU ?
> I think this restriction is added by RvR patch set, right ?
> I'm sorry that I cannot catch the whole changes..

Kame-san:  The restriction to put the page back to the LRU via
putback_lru_page() with the page locked does come from the unevictable
page infrastructure.  Both page migration and vmscan can hold the page
isolated from the LRU, but unlocked, for quite some time.  During this
time, a page can become nonreclaimable [or unevictable] or a
nonreclaimable page can become reclaimable.  It's OK if an unevictable
pages gets on on the regular LRU lists, because we'll detect it and
"cull" it if/when vmscan attempts to reclaim it.  However, if a
reclaimable page gets onto the unevictable LRU list, we may never get it
off, except via manual scan.  Rik doesn't think we need the manual scan,
so we've been very careful to avoid conditions where we could "leak" a
reclaimable page permantently onto the unevictable list.  Kosaki-san
found several scenarios where this could happen unless we check, under
page lock, the unevictable conditions when putting these pages back on
the LRU.

> 
> Anyway, IMHO, lock <-> unlock should be visible as a pair as much as possible.

I've considered modifying putback_lru_page() not to unlock/put the page
when mapping == NULL and count == 1.  Then all of the callers would have
to remember this state, drop the lock and call put page themselves.  I
think this would duplicate code and look ugly, but if we need to do
that, I guess we'll do it.

Regards,
Lee
> 
> Thanks,
> -Kame
> 
> >     I want to balance the put_page() from isolate_lru_page() here for vmscan
> >     and, e.g., page migration rather than requiring explicit checks of the
> >     page_mapping() and explicit put_page() in these areas.  However, the page
> >     could be truncated while one of these subsystems holds it isolated from
> >     the LRU.  So, need to handle this case.  Callers of putback_lru_page()
> >     need to be aware of this and only call it with a page with NULL
> >     page_mapping() when they will no longer reference the page afterwards.
> >     This is the case for vmscan and page migration.
> > 
> > 2)  m[un]lock_vma_page() already will not be called for page with NULL
> >     mapping.  Added VM_BUG_ON() to assert this.
> > 
> > 3)  modified clear_page_lock() to skip the isolate/putback shuffle for
> >     pages with NULL mapping, as they are being truncated/freed.  Thus,
> >     any future callers of clear_page_lock() need not be concerned about
> >     the putback_lru_page() semantics for truncated pages.
> > 
> > Signed-off-by: Lee Schermerhorn <lee.schermerhorn@hp.com>
> > 
> >  mm/mlock.c  |   29 +++++++++++++++++++----------
> >  mm/vmscan.c |   12 +++++++-----
> >  2 files changed, 26 insertions(+), 15 deletions(-)
> > 
> > Index: linux-2.6.26-rc5-mm3/mm/mlock.c
> > ===================================================================
> > --- linux-2.6.26-rc5-mm3.orig/mm/mlock.c	2008-06-12 11:42:59.000000000 -0400
> > +++ linux-2.6.26-rc5-mm3/mm/mlock.c	2008-06-13 09:47:14.000000000 -0400
> > @@ -59,27 +59,33 @@ void __clear_page_mlock(struct page *pag
> >  
> >  	dec_zone_page_state(page, NR_MLOCK);
> >  	count_vm_event(NORECL_PGCLEARED);
> > -	if (!isolate_lru_page(page)) {
> > -		putback_lru_page(page);
> > -	} else {
> > -		/*
> > -		 * Page not on the LRU yet.  Flush all pagevecs and retry.
> > -		 */
> > -		lru_add_drain_all();
> > -		if (!isolate_lru_page(page))
> > +	if (page->mapping) {	/* truncated ? */
> > +		if (!isolate_lru_page(page)) {
> >  			putback_lru_page(page);
> > -		else if (PageUnevictable(page))
> > -			count_vm_event(NORECL_PGSTRANDED);
> > +		} else {
> > +			/*
> > +			 * Page not on the LRU yet.
> > +			 * Flush all pagevecs and retry.
> > +			 */
> > +			lru_add_drain_all();
> > +			if (!isolate_lru_page(page))
> > +				putback_lru_page(page);
> > +			else if (PageUnevictable(page))
> > +				count_vm_event(NORECL_PGSTRANDED);
> > +		}
> >  	}
> >  }
> >  
> >  /*
> >   * Mark page as mlocked if not already.
> >   * If page on LRU, isolate and putback to move to unevictable list.
> > + *
> > + * Called with page locked and page_mapping() != NULL.
> >   */
> >  void mlock_vma_page(struct page *page)
> >  {
> >  	BUG_ON(!PageLocked(page));
> > +	VM_BUG_ON(!page_mapping(page));
> >  
> >  	if (!TestSetPageMlocked(page)) {
> >  		inc_zone_page_state(page, NR_MLOCK);
> > @@ -92,6 +98,8 @@ void mlock_vma_page(struct page *page)
> >  /*
> >   * called from munlock()/munmap() path with page supposedly on the LRU.
> >   *
> > + * Called with page locked and page_mapping() != NULL.
> > + *
> >   * Note:  unlike mlock_vma_page(), we can't just clear the PageMlocked
> >   * [in try_to_munlock()] and then attempt to isolate the page.  We must
> >   * isolate the page to keep others from messing with its unevictable
> > @@ -110,6 +118,7 @@ void mlock_vma_page(struct page *page)
> >  static void munlock_vma_page(struct page *page)
> >  {
> >  	BUG_ON(!PageLocked(page));
> > +	VM_BUG_ON(!page_mapping(page));
> >  
> >  	if (TestClearPageMlocked(page)) {
> >  		dec_zone_page_state(page, NR_MLOCK);
> > Index: linux-2.6.26-rc5-mm3/mm/vmscan.c
> > ===================================================================
> > --- linux-2.6.26-rc5-mm3.orig/mm/vmscan.c	2008-06-12 11:39:09.000000000 -0400
> > +++ linux-2.6.26-rc5-mm3/mm/vmscan.c	2008-06-13 09:44:44.000000000 -0400
> > @@ -1,4 +1,4 @@
> > -/*
> > + /*
> >   *  linux/mm/vmscan.c
> >   *
> >   *  Copyright (C) 1991, 1992, 1993, 1994  Linus Torvalds
> > @@ -488,6 +488,9 @@ int remove_mapping(struct address_space 
> >   * lru_lock must not be held, interrupts must be enabled.
> >   * Must be called with page locked.
> >   *
> > + * If page truncated [page_mapping() == NULL] and we hold the last reference,
> > + * the page will be freed here.  For vmscan and page migration.
> > + *
> >   * return 1 if page still locked [not truncated], else 0
> >   */
> >  int putback_lru_page(struct page *page)
> > @@ -502,12 +505,11 @@ int putback_lru_page(struct page *page)
> >  	lru = !!TestClearPageActive(page);
> >  	was_unevictable = TestClearPageUnevictable(page); /* for page_evictable() */
> >  
> > -	if (unlikely(!page->mapping)) {
> > +	if (unlikely(!page->mapping && page_count(page) == 1)) {
> >  		/*
> > -		 * page truncated.  drop lock as put_page() will
> > -		 * free the page.
> > +		 * page truncated and we hold last reference.
> > +		 * drop lock as put_page() will free the page.
> >  		 */
> > -		VM_BUG_ON(page_count(page) != 1);
> >  		unlock_page(page);
> >  		ret = 0;
> >  	} else if (page_evictable(page, NULL)) {
> > 
> > 
> > 
> 

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