linux-kernel - Re: [PATCH 4/4] anonvma: when setting up page->mapping, we need to pick the _oldest

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Date:	Mon, 12 Apr 2010 18:08:04 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Johannes Weiner <hannes@...xchg.org>
cc:	Borislav Petkov <bp@...en8.de>, Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.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 4/4] anonvma: when setting up page->mapping, we need to
 pick the _oldest_ anonvma



On Tue, 13 Apr 2010, Johannes Weiner wrote:
> 
> Would you mind pasting that nice description of the error case from your
> other email into that changelog?  I skimmed over the description but when
> I read this patch several hours later, I had to go back to that previous
> email to fully make sense of it.

It now looks like this..

		Linus
---
From: Linus Torvalds <torvalds@...ux-foundation.org>
Date: Mon, 12 Apr 2010 12:44:29 -0700
Subject: [PATCH 4/4] anonvma: when setting up page->mapping, we need to pick the _oldest_ anonvma

Otherwise we might be mapping in a page in a new mapping, but that page
(through the swapcache) would later be mapped into an old mapping too.
The page->mapping must be the case that works for everybody, not just
the mapping that happened to page it in first.

Here's the scenario:

 - page gets allocated/mapped by process A. Let's call the anon_vma we
   associate the page with 'A' to keep it easy to track.

 - Process A forks, creating process B. The anon_vma in B is 'B', and has
   a chain that looks like 'B' -> 'A'. Everything is fine.

 - Swapping happens. The page (with mapping pointing to 'A') gets swapped
   out (perhaps not to disk - it's enough to assume that it's just not
   mapped any more, and lives entirely in the swap-cache)

 - Process B pages it in, which goes like this:

        do_swap_page ->
          page = lookup_swap_cache(entry);
         ...
          set_pte_at(mm, address, page_table, pte);
          page_add_anon_rmap(page, vma, address);

   And think about what happens here!

   In particular, what happens is that this will now be the "first"
   mapping of that page, so page_add_anon_rmap() used to do

        if (first)
                __page_set_anon_rmap(page, vma, address);

   and notice what anon_vma it will use? It will use the anon_vma for
   process B!

   What happens then? Trivial: process 'A' also pages it in (nothing
   happens, it's not the first mapping), and then process 'B' execve's
   or exits or unmaps, making anon_vma B go away.

   End result: process A has a page that points to anon_vma B, but
   anon_vma B does not exist any more.  This can go on forever.  Forget
   about RCU grace periods, forget about locking, forget anything like
   that.  The bug is simply that page->mapping points to an anon_vma
   that was correct at one point, but was _not_ the one that was shared
   by all users of that possible mapping.

Changing it to always use the deepest anon_vma in the anonvma chain gets
us to the safest model.

This can be improved in certain cases: if we know the page is private to
just this particular mapping (for example, it's a new page, or it is the
only swapcache entry), we could pick the top (most specific) anon_vma.

But that's a future optimization. Make it _work_ reliably first.

Reviewed-by: Rik van Riel <riel@...hat.com>
Acked-by: Johannes Weiner <hannes@...xchg.org>
Tested-by: Borislav Petkov <bp@...en8.de> [ "What do you know, I think you fixed it!" ]
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
---
 mm/rmap.c |   15 +++++++++++++--
 1 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index ee97d38..4bad326 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -734,9 +734,20 @@ void page_move_anon_rmap(struct page *page,
 static void __page_set_anon_rmap(struct page *page,
 	struct vm_area_struct *vma, unsigned long address)
 {
-	struct anon_vma *anon_vma = vma->anon_vma;
+	struct anon_vma_chain *avc;
+	struct anon_vma *anon_vma;
+
+	BUG_ON(!vma->anon_vma);
+
+	/*
+	 * We must use the _oldest_ possible anon_vma for the page mapping!
+	 *
+	 * So take the last AVC chain entry in the vma, which is the deepest
+	 * ancestor, and use the anon_vma from that.
+	 */
+	avc = list_entry(vma->anon_vma_chain.prev, struct anon_vma_chain, same_vma);
+	anon_vma = avc->anon_vma;
 
-	BUG_ON(!anon_vma);
 	anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
 	page->mapping = (struct address_space *) anon_vma;
 	page->index = linear_page_index(vma, address);
-- 
1.7.1.rc1.dirty

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