linux-kernel - [PATCH v8 16/17] iov_iter: Introduce nofault flag to disable page faults

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Date:   Tue, 19 Oct 2021 15:42:03 +0200
From:   Andreas Gruenbacher <agruenba@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Paul Mackerras <paulus@...abs.org>, Jan Kara <jack@...e.cz>,
        Matthew Wilcox <willy@...radead.org>, cluster-devel@...hat.com,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        ocfs2-devel@....oracle.com, kvm-ppc@...r.kernel.org,
        linux-btrfs@...r.kernel.org,
        Andreas Gruenbacher <agruenba@...hat.com>
Subject: [PATCH v8 16/17] iov_iter: Introduce nofault flag to disable page faults

Introduce a new nofault flag to indicate to iov_iter_get_pages not to
fault in user pages.

This is implemented by passing the FOLL_NOFAULT flag to get_user_pages,
which causes get_user_pages to fail when it would otherwise fault in a
page. We'll use the ->nofault flag to prevent iomap_dio_rw from faulting
in pages when page faults are not allowed.

Signed-off-by: Andreas Gruenbacher <agruenba@...hat.com>
---
 include/linux/uio.h |  1 +
 lib/iov_iter.c      | 20 +++++++++++++++-----
 2 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/include/linux/uio.h b/include/linux/uio.h
index 25d1c24fd829..6350354f97e9 100644
--- a/include/linux/uio.h
+++ b/include/linux/uio.h
@@ -35,6 +35,7 @@ struct iov_iter_state {
 
 struct iov_iter {
 	u8 iter_type;
+	bool nofault;
 	bool data_source;
 	size_t iov_offset;
 	size_t count;
diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index ac9a87e727a3..66a740e6e153 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -513,6 +513,7 @@ void iov_iter_init(struct iov_iter *i, unsigned int direction,
 	WARN_ON(direction & ~(READ | WRITE));
 	*i = (struct iov_iter) {
 		.iter_type = ITER_IOVEC,
+		.nofault = false,
 		.data_source = direction,
 		.iov = iov,
 		.nr_segs = nr_segs,
@@ -1527,13 +1528,17 @@ ssize_t iov_iter_get_pages(struct iov_iter *i,
 		return 0;
 
 	if (likely(iter_is_iovec(i))) {
+		unsigned int gup_flags = 0;
 		unsigned long addr;
 
+		if (iov_iter_rw(i) != WRITE)
+			gup_flags |= FOLL_WRITE;
+		if (i->nofault)
+			gup_flags |= FOLL_NOFAULT;
+
 		addr = first_iovec_segment(i, &len, start, maxsize, maxpages);
 		n = DIV_ROUND_UP(len, PAGE_SIZE);
-		res = get_user_pages_fast(addr, n,
-				iov_iter_rw(i) != WRITE ?  FOLL_WRITE : 0,
-				pages);
+		res = get_user_pages_fast(addr, n, gup_flags, pages);
 		if (unlikely(res <= 0))
 			return res;
 		return (res == n ? len : res * PAGE_SIZE) - *start;
@@ -1649,15 +1654,20 @@ ssize_t iov_iter_get_pages_alloc(struct iov_iter *i,
 		return 0;
 
 	if (likely(iter_is_iovec(i))) {
+		unsigned int gup_flags = 0;
 		unsigned long addr;
 
+		if (iov_iter_rw(i) != WRITE)
+			gup_flags |= FOLL_WRITE;
+		if (i->nofault)
+			gup_flags |= FOLL_NOFAULT;
+
 		addr = first_iovec_segment(i, &len, start, maxsize, ~0U);
 		n = DIV_ROUND_UP(len, PAGE_SIZE);
 		p = get_pages_array(n);
 		if (!p)
 			return -ENOMEM;
-		res = get_user_pages_fast(addr, n,
-				iov_iter_rw(i) != WRITE ?  FOLL_WRITE : 0, p);
+		res = get_user_pages_fast(addr, n, gup_flags, p);
 		if (unlikely(res <= 0)) {
 			kvfree(p);
 			*pages = NULL;
-- 
2.26.3