linux-kernel - [PATCH AUTOSEL for 4.14 028/100] s390: fix alloc_pgste check in init_new

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Message-ID: <20180124041414.32065-28-alexander.levin@microsoft.com>
Date:   Wed, 24 Jan 2018 04:14:37 +0000
From:   Sasha Levin <Alexander.Levin@...rosoft.com>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
CC:     Martin Schwidefsky <schwidefsky@...ibm.com>,
        Sasha Levin <Alexander.Levin@...rosoft.com>
Subject: [PATCH AUTOSEL for 4.14 028/100] s390: fix alloc_pgste check in
 init_new_context again

From: Martin Schwidefsky <schwidefsky@...ibm.com>

[ Upstream commit 53c4ab70c11c3ba1b9e3caa8e8c17e9c16d9cbc0 ]

git commit badb8bb983e9 "fix alloc_pgste check in init_new_context" fixed
the problem of 'current->mm == NULL' in init_new_context back in 2011.

git commit 3eabaee998c7 "KVM: s390: allow sie enablement for multi-
threaded programs" completely removed the check against alloc_pgste.

git commit 23fefe119ceb "s390/kvm: avoid global config of vm.alloc_pgste=1"
re-added a check against the alloc_pgste flag but without the required
check for current->mm != NULL.

For execve() called by a kernel thread init_new_context() reads from
((struct mm_struct *) NULL)->context.alloc_pgste to decide between
2K vs 4K page tables. If the bit happens to be set for the init process
it will be created with large page tables. This decision is inherited by
all the children of init, this waste quite some memory.

Re-add the check for 'current->mm != NULL'.

Fixes: 23fefe119ceb ("s390/kvm: avoid global config of vm.alloc_pgste=1")
Signed-off-by: Martin Schwidefsky <schwidefsky@...ibm.com>
Signed-off-by: Sasha Levin <alexander.levin@...rosoft.com>
---
 arch/s390/include/asm/mmu_context.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/s390/include/asm/mmu_context.h b/arch/s390/include/asm/mmu_context.h
index 43607bb12cc2..a6cc744ff5fb 100644
--- a/arch/s390/include/asm/mmu_context.h
+++ b/arch/s390/include/asm/mmu_context.h
@@ -28,7 +28,7 @@ static inline int init_new_context(struct task_struct *tsk,
 #ifdef CONFIG_PGSTE
 	mm->context.alloc_pgste = page_table_allocate_pgste ||
 		test_thread_flag(TIF_PGSTE) ||
-		current->mm->context.alloc_pgste;
+		(current->mm && current->mm->context.alloc_pgste);
 	mm->context.has_pgste = 0;
 	mm->context.use_skey = 0;
 	mm->context.use_cmma = 0;
-- 
2.11.0