linux-kernel - [PATCH -next] mm,vmacache: also flush cache for VM

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Date:	Mon, 10 Mar 2014 12:56:15 -0700
From:	Davidlohr Bueso <davidlohr@...com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Michel Lespinasse <walken@...gle.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Davidlohr Bueso <davi@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: [PATCH -next] mm,vmacache: also flush cache for VM_CLONE

Oleg found that there is a potential race if we don't flush the task
for threads (VM_CLONE):

"Suppose we have a task T1 which has the valid vmacache,
T1->vmacache_seqnum == T1->mm->vmacache_seqnum == 0. Suppose it sleeps a lot.

Suppose that its subthread T2 does a lot munmap's, finally mm->vmacache_seqnum
becomes zero again and T2 calls vmacache_flush_all().

T1 wakes up and does clone(CLONE_VM). The new thread T3 gets the copy
of T2's ->vmacache_seqnum and ->vmacache[].

T2 continues, vmacache_flush_all() finds T1 and does vmacache_flush(T1).

But the new thread T3 is not on the list yet, vmacache_flush_all() can't
find it.

So T3 will run with vmacache_valid() == T (till the next invalidate(mm)
of course) but its ->vmacache[] points to nowhere."

Address this by moving the flush call into copy_mm(), instead of only
having it in dup_mm().

Signed-off-by: Davidlohr Bueso <davidlohr@...com>
---
 kernel/fork.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 3e02737..45b6241 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -841,9 +841,6 @@ static struct mm_struct *dup_mm(struct task_struct *tsk)
 	if (mm->binfmt && !try_module_get(mm->binfmt->module))
 		goto free_pt;
 
-	/* initialize the new vmacache entries */
-	vmacache_flush(tsk);
-
 	return mm;
 
 free_pt:
@@ -887,6 +884,9 @@ static int copy_mm(unsigned long clone_flags, struct task_struct *tsk)
 	if (!oldmm)
 		return 0;
 
+	/* initialize the new vmacache entries */
+	vmacache_flush(tsk);
+
 	if (clone_flags & CLONE_VM) {
 		atomic_inc(&oldmm->mm_users);
 		mm = oldmm;
-- 
1.8.1.4



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