linux-kernel - Re: [PATCH -v2] rmap: make anon_vma_prepare link in all the anon

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Message-ID: <alpine.LFD.2.00.1004101101180.3558@i5.linux-foundation.org>
Date:	Sat, 10 Apr 2010 11:21:39 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Borislav Petkov <bp@...en8.de>
cc:	Johannes Weiner <hannes@...xchg.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Nick Piggin <npiggin@...e.de>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	sgunderson@...foot.com
Subject: Re: [PATCH -v2] rmap: make anon_vma_prepare link in all the anon_vmas
 of a mergeable VMA

On Sat, 10 Apr 2010, Linus Torvalds wrote:
> On Sat, 10 Apr 2010, Borislav Petkov wrote:
> > 
> > And I got an oops again, this time the #GP from couple of days ago.
> 
> Oh damn. So the list corruption really does happen still.

Ho humm.

Maybe I'm crazy, but something started bothering me. And I started 
wondering: when is the 'page->mapping' of an anonymous page actually 
cleared?

The thing is, the mapping of an anonymous page is actually cleared only 
when the page is _freed_, in "free_hot_cold_page()". 

Now, let's think about that. And in particular, let's think about how that 
relates to the freeing of the 'anon_vma' that the page->mapping points to. 

The way the anon_vma is freed is when the mapping is torn down, and we do 
roughly:

	tlb = tlb_gather_mmu(mm,..)
	..
	unmap_vmas(&tlb, vma ..
	..
	free_pgtables()
	..
	tlb_finish_mmu(tlb, start, end);

and we actually unmap all the pages in "unmap_vmas()", and then _after_ 
unmapping all the pages we do the "unlink_anon_vmas(vma);" in 
"free_pgtables()". Fine so far - the anon_vma stay around until after the 
page has been happily unmapped.

But "unmapped all the pages" is _not_ actually the same as "free'd all the 
pages". The actual _freeing_ of the page happens generally in 
tlb_finish_mmu(), because we can free the page only after we've flushed 
any TLB entries.

So what we have in that tlb_gather structure is a list of _pending_ pages 
to be freed, while we already actually free'd the anon_vmas earlier!

Now, the thing is, tlb_gather_mmu() begins a preempt-safe region (because 
we use a per-cpu variable), but as far as I can tell it is _not_ an 
RCU-safe region.

So I think we might actually get a real RCU freeing event while this all
happens. So now the 'anon_vma' that 'page->mapping' points to has not just 
been released back to the SLUB caches, the page itself might have been 
released too.

I dunno. Does the above sound at all sane? Or am I just raving?

Something hacky like the above might fix it if I'm not just raving. I 
really might be missing something here.

		Linus

---
 include/asm-generic/tlb.h |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
index e43f976..2678118 100644
--- a/include/asm-generic/tlb.h
+++ b/include/asm-generic/tlb.h
@@ -14,6 +14,7 @@
 #define _ASM_GENERIC__TLB_H

 #include <linux/swap.h>
+#include <linux/rcupdate.h>
 #include <asm/pgalloc.h>
 #include <asm/tlbflush.h>

@@ -62,6 +63,7 @@ tlb_gather_mmu(struct mm_struct *mm, unsigned int full_mm_flush)

 	tlb->fullmm = full_mm_flush;

+	rcu_read_lock();
 	return tlb;
 }

@@ -90,6 +92,7 @@ tlb_finish_mmu(struct mmu_gather *tlb, unsigned long start, unsigned long end)
 	/* keep the page table cache within bounds */
 	check_pgt_cache();

+	rcu_read_unlock();
 	put_cpu_var(mmu_gathers);
 }

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