linux-kernel - [PATCH 1/2] x86: mmu.h: move mm_context_t::lock member inside CONFIG_MODIFY_LDT

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Message-Id: <20190828193836.16791-1-linux@rasmusvillemoes.dk>
Date:   Wed, 28 Aug 2019 21:38:35 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org
Cc:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-kernel@...r.kernel.org
Subject: [PATCH 1/2] x86: mmu.h: move mm_context_t::lock member inside CONFIG_MODIFY_LDT_SYSCALL

The placement of the lock member in mm_context_t suggests that it is
used to protect the vdso* members, but AFAICT, it is only ever used
under #ifdef CONFIG_MODIFY_LDT_SYSCALL. So guarding the member by the
same config option is a cheap way to reduce sizeof(mm_struct) by 32
bytes (only for !CONFIG_MODIFY_LDT_SYSCALL kernels, of course).

Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
---
 arch/x86/include/asm/mmu.h         | 11 ++++++++---
 arch/x86/include/asm/mmu_context.h |  3 +--
 2 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/mmu.h b/arch/x86/include/asm/mmu.h
index e78c7db87801..b1bb47a3577b 100644
--- a/arch/x86/include/asm/mmu.h
+++ b/arch/x86/include/asm/mmu.h
@@ -30,14 +30,13 @@ typedef struct {
 #ifdef CONFIG_MODIFY_LDT_SYSCALL
 	struct rw_semaphore	ldt_usr_sem;
 	struct ldt_struct	*ldt;
+	struct mutex		lock;
 #endif
-
 #ifdef CONFIG_X86_64
 	/* True if mm supports a task running in 32 bit compatibility mode. */
 	unsigned short ia32_compat;
 #endif
 
-	struct mutex lock;
 	void __user *vdso;			/* vdso base address */
 	const struct vdso_image *vdso_image;	/* vdso image in use */
 
@@ -56,10 +55,16 @@ typedef struct {
 #endif
 } mm_context_t;
 
+#ifdef CONFIG_MODIFY_LDT_SYSCALL
+#define INIT_MM_CONTEXT_LOCK(mm) .lock = __MUTEX_INITIALIZER(mm.context.lock),
+#else
+#define INIT_MM_CONTEXT_LOCK(mm)
+#endif
+
 #define INIT_MM_CONTEXT(mm)						\
 	.context = {							\
 		.ctx_id = 1,						\
-		.lock = __MUTEX_INITIALIZER(mm.context.lock),		\
+		INIT_MM_CONTEXT_LOCK(mm)				\
 	}
 
 void leave_mm(int cpu);
diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index 9024236693d2..ac8e3ef8a774 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -82,6 +82,7 @@ static inline void init_new_context_ldt(struct mm_struct *mm)
 {
 	mm->context.ldt = NULL;
 	init_rwsem(&mm->context.ldt_usr_sem);
+	mutex_init(&mm->context.lock);
 }
 int ldt_dup_context(struct mm_struct *oldmm, struct mm_struct *mm);
 void destroy_context_ldt(struct mm_struct *mm);
@@ -186,8 +187,6 @@ void enter_lazy_tlb(struct mm_struct *mm, struct task_struct *tsk);
 static inline int init_new_context(struct task_struct *tsk,
 				   struct mm_struct *mm)
 {
-	mutex_init(&mm->context.lock);
-
 	mm->context.ctx_id = atomic64_inc_return(&last_mm_ctx_id);
 	atomic64_set(&mm->context.tlb_gen, 0);
 
-- 
2.20.1