linux-kernel - [PATCH v6 14/15] x86/clear_page: Introduce clear

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Message-Id: <20250902080816.3715913-15-ankur.a.arora@oracle.com>
Date: Tue,  2 Sep 2025 01:08:15 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org
Cc: akpm@...ux-foundation.org, david@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, mingo@...hat.com,
        mjguzik@...il.com, luto@...nel.org, peterz@...radead.org,
        acme@...nel.org, namhyung@...nel.org, tglx@...utronix.de,
        willy@...radead.org, raghavendra.kt@....com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com,
        ankur.a.arora@...cle.com
Subject: [PATCH v6 14/15] x86/clear_page: Introduce clear_pages()

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.

Define clear_user_pages() while at it.

Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
---
 arch/x86/include/asm/page_64.h | 33 +++++++++++++++++++++++++--------
 1 file changed, 25 insertions(+), 8 deletions(-)

diff --git a/arch/x86/include/asm/page_64.h b/arch/x86/include/asm/page_64.h
index 17b6ae89e211..289b31a4c910 100644
--- a/arch/x86/include/asm/page_64.h
+++ b/arch/x86/include/asm/page_64.h
@@ -43,8 +43,11 @@ 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.
- * @page: address of kernel page
+ * clear_page() - clear a page range using a kernel virtual address.
+ * @addr: start address
+ * @npages: number of pages
+ *
+ * Assumes that (@addr, +@...ges) references a kernel region.
  *
  * Switch between three implementations of page clearing based on CPU
  * capabilities:
@@ -65,21 +68,35 @@ void memzero_page_aligned_unrolled(void *addr, u64 len);
  *
  * Does absolutely no exception handling.
  */
-static inline void clear_page(void *page)
+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
-	 * below clobbers @page, so we perform unpoisoning before it.
+	 * Clean up KMSAN metadata for the pages being cleared. The assembly call
+	 * below clobbers @addr, so we perform unpoisoning before it.
 	 */
-	kmsan_unpoison_memory(page, len);
+	kmsan_unpoison_memory(addr, len);
 	asm volatile(ALTERNATIVE_2("call memzero_page_aligned_unrolled",
 				   "shrq $3, %%rcx; rep stosq", X86_FEATURE_REP_GOOD,
 				   "rep stosb", X86_FEATURE_ERMS)
-			: "+c" (len), "+D" (page), ASM_CALL_CONSTRAINT
+			: "+c" (len), "+D" (addr), ASM_CALL_CONSTRAINT
 			: "a" (0)
 			: "cc", "memory");
 }
+#define clear_pages clear_pages
+
+struct page;
+static inline void clear_user_pages(void *page, unsigned long vaddr,
+				    struct page *pg, unsigned int npages)
+{
+	clear_pages(page, npages);
+}
+#define clear_user_pages clear_user_pages
+
+static inline void clear_page(void *addr)
+{
+	clear_pages(addr, 1);
+}
 
 void copy_page(void *to, void *from);
 KCFI_REFERENCE(copy_page);
-- 
2.31.1