linux-kernel - Re: mm: ptl is not bloated if it fits in pointer

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Message-Id: <20131216135005.AC5FDE0090@blue.fi.intel.com>
Date:	Mon, 16 Dec 2013 15:50:05 +0200 (EET)
From:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Hugh Dickins <hughd@...gle.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: mm: ptl is not bloated if it fits in pointer

Peter Zijlstra wrote:
> On Mon, Dec 16, 2013 at 01:04:13AM -0800, Hugh Dickins wrote:
> > It's silly to force the 64-bit CONFIG_GENERIC_LOCKBREAK architectures
> > to kmalloc eight bytes for an indirect page table lock: the lock needs
> > to fit in the space that a pointer to it would occupy, not into an int.
> 
> Ah, no. A spinlock is very much assumed to be 32bit, any spinlock that's
> bigger than that is bloated.
> 
> For the page-frame case we do indeed not care about the strict 32bit but
> more about not being larger than a pointer, however there are already
> other users.
> 
> See for instance include/linux/lockref.h and lib/lockref.c, they very
> much require the spinlock to be 32bit and the below would break that.

What about this instead? Smoke-tested.

>From d0243ba6bf462a7c0ae4290e1c0dbdae22eaa2a6 Mon Sep 17 00:00:00 2001
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Date: Mon, 16 Dec 2013 15:36:31 +0200
Subject: [PATCH] mm: do not allocate page->ptl dynamically, if spinlock_t fits
 to long

In struct page we have enough space to fit long-size page->ptl
there, but we use dynamically-allocated page->ptl if
size(spinlock_t) > sizeof(int). It hurts 64-bit architectures with
CONFIG_GENERIC_LOCKBREAK, where sizeof(spinlock_t) == 8, but it
easily fits into struct page.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
---
 include/linux/lockref.h  | 2 +-
 include/linux/mm.h       | 6 +++---
 include/linux/mm_types.h | 3 ++-
 kernel/bounds.c          | 2 +-
 mm/memory.c              | 2 +-
 5 files changed, 8 insertions(+), 7 deletions(-)

diff --git a/include/linux/lockref.h b/include/linux/lockref.h
index c8929c3832db..4bfde0e99ed5 100644
--- a/include/linux/lockref.h
+++ b/include/linux/lockref.h
@@ -19,7 +19,7 @@
 
 #define USE_CMPXCHG_LOCKREF \
 	(IS_ENABLED(CONFIG_ARCH_USE_CMPXCHG_LOCKREF) && \
-	 IS_ENABLED(CONFIG_SMP) && !BLOATED_SPINLOCKS)
+	 IS_ENABLED(CONFIG_SMP) && SPINLOCK_SIZE <= 4)
 
 struct lockref {
 	union {
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 1cedd000cf29..35527173cf50 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1317,7 +1317,7 @@ static inline pmd_t *pmd_alloc(struct mm_struct *mm, pud_t *pud, unsigned long a
 #endif /* CONFIG_MMU && !__ARCH_HAS_4LEVEL_HACK */
 
 #if USE_SPLIT_PTE_PTLOCKS
-#if BLOATED_SPINLOCKS
+#if ALLOC_SPLIT_PTLOCKS
 extern bool ptlock_alloc(struct page *page);
 extern void ptlock_free(struct page *page);
 
@@ -1325,7 +1325,7 @@ static inline spinlock_t *ptlock_ptr(struct page *page)
 {
 	return page->ptl;
 }
-#else /* BLOATED_SPINLOCKS */
+#else /* ALLOC_SPLIT_PTLOCKS */
 static inline bool ptlock_alloc(struct page *page)
 {
 	return true;
@@ -1339,7 +1339,7 @@ static inline spinlock_t *ptlock_ptr(struct page *page)
 {
 	return &page->ptl;
 }
-#endif /* BLOATED_SPINLOCKS */
+#endif /* ALLOC_SPLIT_PTLOCKS */
 
 static inline spinlock_t *pte_lockptr(struct mm_struct *mm, pmd_t *pmd)
 {
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index bd299418a934..494b328c2a61 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -26,6 +26,7 @@ struct address_space;
 #define USE_SPLIT_PTE_PTLOCKS	(NR_CPUS >= CONFIG_SPLIT_PTLOCK_CPUS)
 #define USE_SPLIT_PMD_PTLOCKS	(USE_SPLIT_PTE_PTLOCKS && \
 		IS_ENABLED(CONFIG_ARCH_ENABLE_SPLIT_PMD_PTLOCK))
+#define ALLOC_SPLIT_PTLOCKS	(SPINLOCK_SIZE > BITS_PER_LONG/8)
 
 /*
  * Each physical page in the system has a struct page associated with
@@ -155,7 +156,7 @@ struct page {
 						 * system if PG_buddy is set.
 						 */
 #if USE_SPLIT_PTE_PTLOCKS
-#if BLOATED_SPINLOCKS
+#if ALLOC_SPLIT_PTLOCKS
 		spinlock_t *ptl;
 #else
 		spinlock_t ptl;
diff --git a/kernel/bounds.c b/kernel/bounds.c
index 5253204afdca..9fd4246b04b8 100644
--- a/kernel/bounds.c
+++ b/kernel/bounds.c
@@ -22,6 +22,6 @@ void foo(void)
 #ifdef CONFIG_SMP
 	DEFINE(NR_CPUS_BITS, ilog2(CONFIG_NR_CPUS));
 #endif
-	DEFINE(BLOATED_SPINLOCKS, sizeof(spinlock_t) > sizeof(int));
+	DEFINE(SPINLOCK_SIZE, sizeof(spinlock_t));
 	/* End of constants */
 }
diff --git a/mm/memory.c b/mm/memory.c
index 5d9025f3b3e1..b6e211b779d0 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4271,7 +4271,7 @@ void copy_user_huge_page(struct page *dst, struct page *src,
 }
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE || CONFIG_HUGETLBFS */
 
-#if USE_SPLIT_PTE_PTLOCKS && BLOATED_SPINLOCKS
+#if ALLOC_SPLIT_PTLOCKS
 bool ptlock_alloc(struct page *page)
 {
 	spinlock_t *ptl;
-- 
 Kirill A. Shutemov
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