[Ocfs2-devel] [PATCH v9 15/17] gup: Introduce FOLL_NOFAULT flag to disable page faults

From: Andreas Gruenbacher <agruenba@redhat.com>
To: cluster-devel@redhat.com
Cc: kvm-ppc@vger.kernel.org, Paul Mackerras <paulus@ozlabs.org>,
	Jan Kara <jack@suse.cz>,
	Andreas Gruenbacher <agruenba@redhat.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	linux-kernel@vger.kernel.org,
	Christoph Hellwig <hch@infradead.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	linux-fsdevel@vger.kernel.org, linux-btrfs@vger.kernel.org,
	Linus Torvalds <torvalds@linux-foundation.org>,
	ocfs2-devel@oss.oracle.com
Subject: [Ocfs2-devel] [PATCH v9 15/17] gup: Introduce FOLL_NOFAULT flag to disable page faults
Date: Tue,  2 Nov 2021 13:29:43 +0100	[thread overview]
Message-ID: <20211102122945.117744-16-agruenba@redhat.com> (raw)
In-Reply-To: <20211102122945.117744-1-agruenba@redhat.com>

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 73a52aba448f..2f0e6b9f8f3b 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2851,7 +2851,8 @@ 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 795f15c410cc..e1c7e4bde11f 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -918,6 +918,8 @@ 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)
@@ -2843,7 +2845,7 @@ 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)
-- 
2.31.1


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