[PATCH 1/2] x86: mmu.h: move mm_context_t::lock member inside CONFIG_MODIFY_LDT_SYSCALL

[PATCH 1/2] x86: mmu.h: move mm_context_t::lock member inside CONFIG_MODIFY_LDT_SYSCALL

[Rasmus Villemoes]

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@rasmusvillemoes.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

[PATCH 2/2] x86: mmu.h: move mm_context_t::ia32_compat member a bit down

[Rasmus Villemoes]

For CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS=n, there's both a
6-byte hole after ia32_compat as well as a 4-byte hole after
perf_rdpmc_allowed. So rearranging things a bit we cut 8 bytes of
sizeof(struct mm_struct).

For a CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS=y kernel, this patch
just moves the 6-byte hole to another place in mm_context_t.

Putting the ia32_compat member after the pkey members is deliberate to
keep the latter two (when present) in the same 4-byte unit.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
---
 arch/x86/include/asm/mmu.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/mmu.h b/arch/x86/include/asm/mmu.h
index b1bb47a3577b..ba3d22fcd507 100644
--- a/arch/x86/include/asm/mmu.h
+++ b/arch/x86/include/asm/mmu.h
@@ -32,10 +32,6 @@ typedef struct {
 	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
 
 	void __user *vdso;			/* vdso base address */
 	const struct vdso_image *vdso_image;	/* vdso image in use */
@@ -49,6 +45,10 @@ typedef struct {
 	u16 pkey_allocation_map;
 	s16 execute_only_pkey;
 #endif
+#ifdef CONFIG_X86_64
+	/* True if mm supports a task running in 32 bit compatibility mode. */
+	unsigned short ia32_compat;
+#endif
 #ifdef CONFIG_X86_INTEL_MPX
 	/* address of the bounds directory */
 	void __user *bd_addr;
-- 
2.20.1

Re: [PATCH 1/2] x86: mmu.h: move mm_context_t::lock member inside CONFIG_MODIFY_LDT_SYSCALL

[Rasmus Villemoes]

On 28/08/2019 21.38, Rasmus Villemoes wrote:
> 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).

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