linux-kernel - [PATCH 5.15 27/33] gup: Introduce FOLL_NOFAULT flag to disable page faults

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Message-Id: <20220429104053.124854665@linuxfoundation.org>
Date:   Fri, 29 Apr 2022 12:42:14 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        Anand Jain <anand.jain@...cle.com>
Subject: [PATCH 5.15 27/33] gup: Introduce FOLL_NOFAULT flag to disable page faults

From: Andreas Gruenbacher <agruenba@...hat.com>

commit 55b8fe703bc51200d4698596c90813453b35ae63 upstream

Introduce a new FOLL_NOFAULT flag that causes get_user_pages to return
-EFAULT when it would otherwise trigger a page fault.  This is roughly
similar to FOLL_FAST_ONLY but available on all architectures, and less
fragile.

Signed-off-by: Andreas Gruenbacher <agruenba@...hat.com>
Signed-off-by: Anand Jain <anand.jain@...cle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 include/linux/mm.h |    3 ++-
 mm/gup.c           |    4 +++-
 2 files changed, 5 insertions(+), 2 deletions(-)

--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2858,7 +2858,8 @@ struct page *follow_page(struct vm_area_
 #define FOLL_FORCE	0x10	/* get_user_pages read/write w/o permission */
 #define FOLL_NOWAIT	0x20	/* if a disk transfer is needed, start the IO
 				 * and return without waiting upon it */
-#define FOLL_POPULATE	0x40	/* fault in page */
+#define FOLL_POPULATE	0x40	/* fault in pages (with FOLL_MLOCK) */
+#define FOLL_NOFAULT	0x80	/* do not fault in pages */
 #define FOLL_HWPOISON	0x100	/* check page is hwpoisoned */
 #define FOLL_NUMA	0x200	/* force NUMA hinting page fault */
 #define FOLL_MIGRATION	0x400	/* wait for page to replace migration entry */
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -943,6 +943,8 @@ static int faultin_page(struct vm_area_s
 	/* mlock all present pages, but do not fault in new pages */
 	if ((*flags & (FOLL_POPULATE | FOLL_MLOCK)) == FOLL_MLOCK)
 		return -ENOENT;
+	if (*flags & FOLL_NOFAULT)
+		return -EFAULT;
 	if (*flags & FOLL_WRITE)
 		fault_flags |= FAULT_FLAG_WRITE;
 	if (*flags & FOLL_REMOTE)
@@ -2868,7 +2870,7 @@ static int internal_get_user_pages_fast(
 
 	if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM |
 				       FOLL_FORCE | FOLL_PIN | FOLL_GET |
-				       FOLL_FAST_ONLY)))
+				       FOLL_FAST_ONLY | FOLL_NOFAULT)))
 		return -EINVAL;
 
 	if (gup_flags & FOLL_PIN)