linux-kernel - [PATCH v10 4/8] highmem: do range clearing in clear_user

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Message-Id: <20251215204922.475324-5-ankur.a.arora@oracle.com>
Date: Mon, 15 Dec 2025 12:49:18 -0800
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@...nel.org, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, mingo@...hat.com,
        mjguzik@...il.com, luto@...nel.org, peterz@...radead.org,
        tglx@...utronix.de, willy@...radead.org, raghavendra.kt@....com,
        chleroy@...nel.org, ioworker0@...il.com, boris.ostrovsky@...cle.com,
        konrad.wilk@...cle.com, ankur.a.arora@...cle.com
Subject: [PATCH v10 4/8] highmem: do range clearing in clear_user_highpages()

Use the range clearing primitive clear_user_pages() when clearing
contiguous pages in clear_user_highpages().

We can safely do that when we have !CONFIG_HIGHMEM and when the
architecture does not have clear_user_highpage.

The first is necessary because not doing intermediate maps for
pages lets contiguous page ranges stay contiguous. The second,
because if the architecture has clear_user_highpage(), it likely
needs flushing magic when clearing the page, magic that we aren't
privy to.

Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
---
Note:
  - reorganized based on the previous two patches. 
  - Removed David's acked-by.

 include/linux/highmem.h | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/include/linux/highmem.h b/include/linux/highmem.h
index 92aa1053c9c1..c6219700569f 100644
--- a/include/linux/highmem.h
+++ b/include/linux/highmem.h
@@ -278,11 +278,28 @@ static inline void clear_user_highpage(struct page *page, unsigned long vaddr)
 static inline void clear_user_highpages(struct page *page, unsigned long vaddr,
 					unsigned int npages)
 {
+
+#if defined(clear_user_highpage) || defined(CONFIG_HIGHMEM)
+	/*
+	 * An architecture defined clear_user_highpage() implies special
+	 * handling is needed.
+	 *
+	 * So we use that or, the generic variant if CONFIG_HIGHMEM is
+	 * enabled.
+	 */
 	do {
 		clear_user_highpage(page, vaddr);
 		vaddr += PAGE_SIZE;
 		page++;
 	} while (--npages);
+#else
+
+	/*
+	 * Prefer clear_user_pages() to allow for architectural optimizations
+	 * when operating on contiguous page ranges.
+	 */
+	clear_user_pages(page_address(page), vaddr, page, npages);
+#endif
 }
 
 #ifndef vma_alloc_zeroed_movable_folio
-- 
2.31.1