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Date: Mon, 31 Aug 2020 16:40:51 +0800
From: Alex Shi <alex.shi@...ux.alibaba.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: Anshuman Khandual <anshuman.khandual@....com>,
David Hildenbrand <david@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Hugh Dickins <hughd@...gle.com>,
Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] mm/pageblock: mitigation cmpxchg false sharing in
pageblock flags
在 2020/8/30 下午6:18, Matthew Wilcox 写道:
> On Sun, Aug 30, 2020 at 06:14:33PM +0800, Alex Shi wrote:
>> +++ b/mm/page_alloc.c
>> @@ -532,9 +536,18 @@ void set_pfnblock_flags_mask(struct page *page, unsigned long flags,
>> mask <<= bitidx;
>> flags <<= bitidx;
>>
>> +#ifdef CONFIG_CPU_V6
>> + byte = (unsigned long)READ_ONCE(bitmap[byte_bitidx]);
>> +#else
>> byte = READ_ONCE(bitmap[byte_bitidx]);
>> +#endif
>> for (;;) {
>> +#ifdef CONFIG_CPU_V6
>> + /* arm v6 has no cmpxchgb function, so still false sharing long word */
>> + old_byte = cmpxchg((unsigned long*)&bitmap[byte_bitidx], byte, (byte & ~mask) | flags);
>> +#else
>> old_byte = cmpxchg(&bitmap[byte_bitidx], byte, (byte & ~mask) | flags);
>> +#endif
>
> Good grief, no. Either come up with an appropriate abstraction or
> abandon this patch. We can't possibly put this kind of ifdef in the
> memory allocator!
>
Hi Matthew,
Thanks a lot for comments! How about the following patch?
>From 5f61b91351461084c5bb410025965a3b4d2f7206 Mon Sep 17 00:00:00 2001
From: Alex Shi <alex.shi@...ux.alibaba.com>
Date: Mon, 31 Aug 2020 15:41:20 +0800
Subject: [PATCH 3/3] mm/armv6: work around armv6 cmpxchg support issue
Armv6 can not simulate cmpxchg1 func, so we have to use cmpxchg4 on it.
arm-linux-gnueabi-ld: mm/page_alloc.o: in function `set_pfnblock_flags_mask':
(.text+0x32b4): undefined reference to `__bad_cmpxchg'
arm-linux-gnueabi-ld: (.text+0x32e0): undefined reference to `__bad_cmpxchg'
arm-linux-gnueabi-ld:drivers/net/ethernet/aquantia/atlantic/hw_atl/hw_atl_b0.o: in function
`hw_atl_b0_get_mac_temp': hw_atl_b0.c:(.text+0x30fc): undefined reference to `__bad_udelay'
Reported-by: kernel test robot <lkp@...el.com>
Signed-off-by: Alex Shi <alex.shi@...ux.alibaba.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: Russell King <linux@...linux.org.uk>
Cc: linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org
Cc: linux-arm-kernel@...ts.infradead.org
---
include/linux/mmzone.h | 15 ++++++++++++---
mm/page_alloc.c | 24 ++++++++++++------------
2 files changed, 24 insertions(+), 15 deletions(-)
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index be676e659fb7..c1bb904bcad8 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -406,6 +406,15 @@ enum zone_type {
#ifndef __GENERATING_BOUNDS_H
+#ifdef CONFIG_CPU_V6
+/* cmpxchg only support 32-bits operands on ARMv6. */
+typedef unsigned long pageblockflags_t;
+#define BITS_PER_FLAGS BITS_PER_LONG
+#else
+typedef unsigned char pageblockflags_t;
+#define BITS_PER_FLAGS BITS_PER_BYTE
+#endif
+
struct zone {
/* Read-mostly fields */
@@ -437,7 +446,7 @@ struct zone {
* Flags for a pageblock_nr_pages block. See pageblock-flags.h.
* In SPARSEMEM, this map is stored in struct mem_section
*/
- unsigned char *pageblock_flags;
+ pageblockflags_t *pageblock_flags;
#endif /* CONFIG_SPARSEMEM */
/* zone_start_pfn == zone_start_paddr >> PAGE_SHIFT */
@@ -1159,7 +1168,7 @@ struct mem_section_usage {
DECLARE_BITMAP(subsection_map, SUBSECTIONS_PER_SECTION);
#endif
/* See declaration of similar field in struct zone */
- unsigned char pageblock_flags[0];
+ pageblockflags_t pageblock_flags[0];
};
void subsection_map_init(unsigned long pfn, unsigned long nr_pages);
@@ -1212,7 +1221,7 @@ struct mem_section {
extern struct mem_section mem_section[NR_SECTION_ROOTS][SECTIONS_PER_ROOT];
#endif
-static inline unsigned char *section_to_usemap(struct mem_section *ms)
+static inline pageblockflags_t *section_to_usemap(struct mem_section *ms)
{
return ms->usage->pageblock_flags;
}
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 60342e764090..9a41c5dc78eb 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -445,7 +445,7 @@ static inline bool defer_init(int nid, unsigned long pfn, unsigned long end_pfn)
#endif
/* Return a pointer to the bitmap storing bits affecting a block of pages */
-static inline unsigned char *get_pageblock_bitmap(struct page *page,
+static inline pageblockflags_t *get_pageblock_bitmap(struct page *page,
unsigned long pfn)
{
#ifdef CONFIG_SPARSEMEM
@@ -474,24 +474,24 @@ static inline int pfn_to_bitidx(struct page *page, unsigned long pfn)
* Return: pageblock_bits flags
*/
static __always_inline
-unsigned char __get_pfnblock_flags_mask(struct page *page,
+pageblockflags_t __get_pfnblock_flags_mask(struct page *page,
unsigned long pfn,
unsigned long mask)
{
- unsigned char *bitmap;
+ pageblockflags_t *bitmap;
unsigned long bitidx, byte_bitidx;
- unsigned char byte;
+ pageblockflags_t byte;
bitmap = get_pageblock_bitmap(page, pfn);
bitidx = pfn_to_bitidx(page, pfn);
- byte_bitidx = bitidx / BITS_PER_BYTE;
- bitidx &= (BITS_PER_BYTE-1);
+ byte_bitidx = bitidx / BITS_PER_FLAGS;
+ bitidx &= (BITS_PER_FLAGS - 1);
byte = bitmap[byte_bitidx];
return (byte >> bitidx) & mask;
}
-unsigned char get_pfnblock_flags_mask(struct page *page, unsigned long pfn,
+pageblockflags_t get_pfnblock_flags_mask(struct page *page, unsigned long pfn,
unsigned long mask)
{
return __get_pfnblock_flags_mask(page, pfn, mask);
@@ -513,17 +513,17 @@ void set_pfnblock_flags_mask(struct page *page, unsigned long flags,
unsigned long pfn,
unsigned long mask)
{
- unsigned char *bitmap;
+ pageblockflags_t *bitmap;
unsigned long bitidx, byte_bitidx;
- unsigned char old_byte, byte;
+ pageblockflags_t old_byte, byte;
- BUILD_BUG_ON(NR_PAGEBLOCK_BITS != BITS_PER_BYTE);
+ BUILD_BUG_ON(NR_PAGEBLOCK_BITS != BITS_PER_FLAGS);
BUILD_BUG_ON(MIGRATE_TYPES > (1 << PB_migratetype_bits));
bitmap = get_pageblock_bitmap(page, pfn);
bitidx = pfn_to_bitidx(page, pfn);
- byte_bitidx = bitidx / BITS_PER_BYTE;
- bitidx &= (BITS_PER_BYTE-1);
+ byte_bitidx = bitidx / BITS_PER_FLAGS;
+ bitidx &= (BITS_PER_FLAGS - 1);
VM_BUG_ON_PAGE(!zone_spans_pfn(page_zone(page), pfn), page);
--
1.8.3.1
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