linux-kernel - [PATCH] mm: fix kernel BUG at huge

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Date:	Sun, 16 Dec 2012 18:56:58 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Mel Gorman <mgorman@...e.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Ingo Molnar <mingo@...nel.org>, Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Paul Turner <pjt@...gle.com>, Hillf Danton <dhillf@...il.com>,
	David Rientjes <rientjes@...gle.com>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Alex Shi <lkml.alex@...il.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Aneesh Kumar <aneesh.kumar@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: [PATCH] mm: fix kernel BUG at huge_memory.c:1474!

Andrea's autonuma-benchmark numa01 hits kernel BUG at huge_memory.c:1474!
in change_huge_pmd called from change_protection from change_prot_numa
from task_numa_work.

That BUG, introduced in the huge zero page commit cad7f613c4d0 ("thp:
change_huge_pmd(): make sure we don't try to make a page writable")
was trying to verify that newprot never adds write permission to an
anonymous huge page; but Automatic NUMA Balancing's 4b10e7d562c9 ("mm:
mempolicy: Implement change_prot_numa() in terms of change_protection()")
adds a new prot_numa path into change_huge_pmd(), which makes no use of
the newprot provided, and may retain the write bit in the pmd.

Just move the BUG_ON(pmd_write(entry)) up into the !prot_numa block.

Signed-off-by: Hugh Dickins <hughd@...gle.com>
---

 mm/huge_memory.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- 2a74dbb9/mm/huge_memory.c	2012-12-16 16:35:08.752441527 -0800
+++ linux/mm/huge_memory.c	2012-12-16 18:21:24.308156970 -0800
@@ -1460,9 +1460,10 @@ int change_huge_pmd(struct vm_area_struc
 	if (__pmd_trans_huge_lock(pmd, vma) == 1) {
 		pmd_t entry;
 		entry = pmdp_get_and_clear(mm, addr, pmd);
-		if (!prot_numa)
+		if (!prot_numa) {
 			entry = pmd_modify(entry, newprot);
-		else {
+			BUG_ON(pmd_write(entry));
+		} else {
 			struct page *page = pmd_page(*pmd);
 
 			/* only check non-shared pages */
@@ -1471,7 +1472,6 @@ int change_huge_pmd(struct vm_area_struc
 				entry = pmd_mknuma(entry);
 			}
 		}
-		BUG_ON(pmd_write(entry));
 		set_pmd_at(mm, addr, pmd, entry);
 		spin_unlock(&vma->vm_mm->page_table_lock);
 		ret = 1;
--
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