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Groups > linux.kernel > #1206164
| From | "Kirill A. Shutemov" <kirill@shutemov.name> |
|---|---|
| Newsgroups | linux.kernel |
| Subject | Re: page-flags behavior on compound pages: a worry |
| Date | 2015-08-12 16:40 +0200 |
| Message-ID | <pWFmb-7bv-47@gated-at.bofh.it> (permalink) |
| References | <p5M7g-35e-17@gated-at.bofh.it> <p5M7h-35e-29@gated-at.bofh.it> <pUkOR-5Rx-1@gated-at.bofh.it> <pUvqX-4KP-35@gated-at.bofh.it> <pUz1w-1yi-9@gated-at.bofh.it> |
| Organization | linux.* mail to news gateway |
On Thu, Aug 06, 2015 at 12:24:22PM -0700, Hugh Dickins wrote:
> > IIUC, the only potentially problematic callsites left are physical memory
> > scanners. This code requires audit. I'll do that.
>
> Please.
I haven't finished the exercise yet. But here's an issue I believe present
in current *Linus* tree:
From e78eec7d7a8c4cba8b5952a997973f7741e704f4 Mon Sep 17 00:00:00 2001
From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Date: Wed, 12 Aug 2015 17:09:16 +0300
Subject: [PATCH] mm: fix potential race in isolate_migratepages_block()
Hugh has pointed that compound_head() call can be unsafe in some context.
There's one example:
CPU0 CPU1
isolate_migratepages_block()
page_count()
compound_head()
!!PageTail() == true
put_page()
tail->first_page = NULL
head = tail->first_page
alloc_pages(__GFP_COMP)
prep_compound_page()
tail->first_page = head
__SetPageTail(p);
!!PageTail() == true
<head == NULL dereferencing>
The race is pure theoretical. I don't it's possible to trigger it in
practice. But who knows.
This can be fixed by avoiding compound_head() in unsafe context.
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
---
mm/compaction.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/mm/compaction.c b/mm/compaction.c
index 86f04e556f96..bec727b700d3 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -787,7 +787,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
* admittedly racy check.
*/
if (!page_mapping(page) &&
- page_count(page) > page_mapcount(page))
+ atomic_read(&page->_count) > page_mapcount(page))
continue;
/* If we already hold the lock, we can skip some rechecking */
--
Kirill A. Shutemov
--
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Back to linux.kernel | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread
page-flags behavior on compound pages: a worry Hugh Dickins <hughd@google.com> - 2015-08-06 06:20 +0200
Re: page-flags behavior on compound pages: a worry "Kirill A. Shutemov" <kirill@shutemov.name> - 2015-08-06 17:40 +0200
Re: page-flags behavior on compound pages: a worry Hugh Dickins <hughd@google.com> - 2015-08-06 21:30 +0200
Re: page-flags behavior on compound pages: a worry Christoph Lameter <cl@linux.com> - 2015-08-06 22:50 +0200
Re: page-flags behavior on compound pages: a worry "Kirill A. Shutemov" <kirill@shutemov.name> - 2015-08-07 17:00 +0200
Re: page-flags behavior on compound pages: a worry Christoph Lameter <cl@linux.com> - 2015-08-07 17:30 +0200
Re: page-flags behavior on compound pages: a worry "Kirill A. Shutemov" <kirill@shutemov.name> - 2015-08-10 13:20 +0200
Re: page-flags behavior on compound pages: a worry Christoph Lameter <cl@linux.com> - 2015-08-10 16:00 +0200
Re: page-flags behavior on compound pages: a worry "Kirill A. Shutemov" <kirill@shutemov.name> - 2015-08-07 17:00 +0200
Re: page-flags behavior on compound pages: a worry Hugh Dickins <hughd@google.com> - 2015-08-13 07:20 +0200
Re: page-flags behavior on compound pages: a worry "Kirill A. Shutemov" <kirill@shutemov.name> - 2015-08-12 16:40 +0200
Re: page-flags behavior on compound pages: a worry Vlastimil Babka <vbabka@suse.cz> - 2015-08-12 16:50 +0200
Re: page-flags behavior on compound pages: a worry Andrew Morton <akpm@linux-foundation.org> - 2015-08-12 23:20 +0200
Re: page-flags behavior on compound pages: a worry "Kirill A. Shutemov" <kirill@shutemov.name> - 2015-08-13 00:30 +0200
Re: page-flags behavior on compound pages: a worry Hugh Dickins <hughd@google.com> - 2015-08-13 06:20 +0200
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