[PATCH v8 5/7] x86/clear_page: Introduce clear_pages()

Ankur Arora
2025-10-27T20:21:47Z

Performance when clearing with string instructions (x86-64-stosq and
similar) can vary significantly based on the chunk-size used.

$ perf bench mem memset -k 4KB -s 4GB -f x86-64-stosq
# Running 'mem/memset' benchmark:
# function 'x86-64-stosq' (movsq-based memset() in arch/x86/lib/memset_64.S)
# Copying 4GB bytes ...

13.748208 GB/sec

$ perf bench mem memset -k 2MB -s 4GB -f x86-64-stosq
# Running 'mem/memset' benchmark:
# function 'x86-64-stosq' (movsq-based memset() in
# arch/x86/lib/memset_64.S)
# Copying 4GB bytes ...

15.067900 GB/sec

$ perf bench mem memset -k 1GB -s 4GB -f x86-64-stosq
# Running 'mem/memset' benchmark:
# function 'x86-64-stosq' (movsq-based memset() in arch/x86/lib/memset_64.S)
# Copying 4GB bytes ...

38.104311 GB/sec

(Both on AMD Milan.)

With a change in chunk-size of 4KB to 1GB, we see the performance go
from 13.7 GB/sec to 38.1 GB/sec. For a chunk-size of 2MB the change isn't
quite as drastic but it is worth adding a clear_page() variant that can
handle contiguous page-extents.

Also define ARCH_PAGE_CONTIG_NR to specify the maximum contiguous page
range that can be zeroed when running under cooperative preemption
models. This limits the worst case preemption latency.

Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
Tested-by: Raghavendra K T <raghavendra.kt@amd.com>
---
arch/x86/include/asm/page_64.h | 26 +++++++++++++++++++++-----
1 file changed, 21 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/page_64.h b/arch/x86/include/asm/page_64.h
index df528cff90ef..efab5dc26e3e 100644
--- a/arch/x86/include/asm/page_64.h
+++ b/arch/x86/include/asm/page_64.h
@@ -43,8 +43,9 @@ extern unsigned long __phys_addr_symbol(unsigned long);
void memzero_page_aligned_unrolled(void *addr, u64 len);

/**
- * clear_page() - clear a page using a kernel virtual address.
- * @addr: address of kernel page
+ * clear_pages() - clear a page range using a kernel virtual address.
+ * @addr: start address of kernel page range
+ * @npages: number of pages
*
* Switch between three implementations of page clearing based on CPU
* capabilities:
@@ -65,11 +66,11 @@ void memzero_page_aligned_unrolled(void *addr, u64 len);
*
* Does absolutely no exception handling.
*/
-static inline void clear_page(void *addr)
+static inline void clear_pages(void *addr, unsigned int npages)
{
- u64 len = PAGE_SIZE;
+ u64 len = npages * PAGE_SIZE;
/*
- * Clean up KMSAN metadata for the page being cleared. The assembly call
+ * Clean up KMSAN metadata for the pages being cleared. The assembly call
* below clobbers @addr, so we perform unpoisoning before it.
*/
kmsan_unpoison_memory(addr, len);
@@ -80,6 +81,21 @@ static inline void clear_page(void *addr)
: "a" (0)
: "cc", "memory");
}
+#define __HAVE_ARCH_CLEAR_PAGES
+
+/*
+ * When running under cooperatively scheduled preemption models limit the
+ * maximum contiguous extent that can be cleared to pages worth 8MB.
+ *
+ * With a clearing BW of ~10GBps, this should result in worst case scheduling
+ * latency of ~1ms.
+ */
+#define ARCH_PAGE_CONTIG_NR (8 << (20 - PAGE_SHIFT))
+
+static inline void clear_page(void *addr)
+{
+ clear_pages(addr, 1);
+}

void copy_page(void *to, void *from);
KCFI_REFERENCE(copy_page);
--
2.43.5