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From:   Andreas Gruenbacher <agruenba@redhat.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@infradead.org>,
        "Darrick J. Wong" <djwong@kernel.org>
Cc:     Jan Kara <jack@suse.cz>, Matthew Wilcox <willy@infradead.org>,
        cluster-devel@redhat.com, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, ocfs2-devel@oss.oracle.com,
        Andreas Gruenbacher <agruenba@redhat.com>
Subject: [PATCH v7 17/19] gup: Introduce FOLL_NOFAULT flag to disable page faults
Date:   Fri, 27 Aug 2021 18:49:24 +0200
Message-Id: <20210827164926.1726765-18-agruenba@redhat.com>
In-Reply-To: <20210827164926.1726765-1-agruenba@redhat.com>
References: <20210827164926.1726765-1-agruenba@redhat.com>
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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@redhat.com>
---
 include/linux/mm.h | 3 ++-
 mm/gup.c           | 4 +++-
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 7ca22e6e694a..958246aa343f 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2850,7 +2850,8 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
 #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 */
diff --git a/mm/gup.c b/mm/gup.c
index 03ab03b68dc7..69056adcc8c9 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -932,6 +932,8 @@ static int faultin_page(struct vm_area_struct *vma,
 	/* 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)
@@ -2857,7 +2859,7 @@ static int internal_get_user_pages_fast(unsigned long start,
 
 	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)
-- 
2.26.3