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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 v5 07/12] gfs2: Fix mmap + page fault deadlocks for buffered I/O
Date: Tue,  3 Aug 2021 21:18:13 +0200	[thread overview]
Message-ID: <20210803191818.993968-8-agruenba@redhat.com> (raw)
In-Reply-To: <20210803191818.993968-1-agruenba@redhat.com>

In the .read_iter and .write_iter file operations, we're accessing
user-space memory while holding the inodes glock.  There's a possibility
that the memory is mapped to the same file, in which case we'd recurse on
the same glock.

More complex scenarios can involve multiple glocks, processes, and even cluster
nodes.

Avoids these kinds of problems by disabling page faults while holding a glock.
If a page fault occurs, we either end up with a partial read or write, or with
-EFAULT if nothing could be read or written.  In that case, we drop the glock,
fault in the requested pages manually, and repeat the operation.

This locking problem in gfs2 was originally reported by Jan Kara.  Linus came
up with the proposal to disable page faults.  Many thanks to Al Viro and
Matthew Wilcox for their feedback as well.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
---
 fs/gfs2/file.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/fs/gfs2/file.c b/fs/gfs2/file.c
index 55ec1cadc9e6..c0f86a28f1bf 100644
--- a/fs/gfs2/file.c
+++ b/fs/gfs2/file.c
@@ -843,6 +843,12 @@ static ssize_t gfs2_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
 	size_t written = 0;
 	ssize_t ret;
 
+	/*
+	 * In this function, we disable page faults when we're holding the
+	 * inode glock while doing I/O.  If a page fault occurs, we drop the
+	 * inode glock, fault in the pages manually, and retry.
+	 */
+
 	if (iocb->ki_flags & IOCB_DIRECT) {
 		ret = gfs2_file_direct_read(iocb, to, &gh);
 		if (likely(ret != -ENOTBLK))
@@ -864,13 +870,20 @@ static ssize_t gfs2_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
 	}
 	ip = GFS2_I(iocb->ki_filp->f_mapping->host);
 	gfs2_holder_init(ip->i_gl, LM_ST_SHARED, 0, &gh);
+retry:
 	ret = gfs2_glock_nq(&gh);
 	if (ret)
 		goto out_uninit;
+	pagefault_disable();
 	ret = generic_file_read_iter(iocb, to);
+	pagefault_enable();
 	if (ret > 0)
 		written += ret;
 	gfs2_glock_dq(&gh);
+	if (unlikely(iov_iter_count(to) && (ret > 0 || ret == -EFAULT)) &&
+	    iter_is_iovec(to) &&
+	    fault_in_iov_iter_writeable(to, SIZE_MAX) != 0)
+		goto retry;
 out_uninit:
 	gfs2_holder_uninit(&gh);
 	return written ? written : ret;
@@ -882,9 +895,22 @@ static ssize_t gfs2_file_buffered_write(struct kiocb *iocb, struct iov_iter *fro
 	struct inode *inode = file_inode(file);
 	ssize_t ret;
 
+	/*
+	 * In this function, we disable page faults when we're holding the
+	 * inode glock while doing I/O.  If a page fault occurs, we drop the
+	 * inode glock, fault in the pages manually, and retry.
+	 */
+
+retry:
 	current->backing_dev_info = inode_to_bdi(inode);
+	pagefault_disable();
 	ret = iomap_file_buffered_write(iocb, from, &gfs2_iomap_ops);
+	pagefault_enable();
 	current->backing_dev_info = NULL;
+	if (unlikely(ret == -EFAULT) &&
+	    iter_is_iovec(from) &&
+	    fault_in_iov_iter_readable(from, SIZE_MAX) != 0)
+		goto retry;
 	return ret;
 }
 
-- 
2.26.3


  parent reply	other threads:[~2021-08-03 19:19 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-03 19:18 [PATCH v5 00/12] gfs2: Fix mmap + page fault deadlocks Andreas Gruenbacher
2021-08-03 19:18 ` [PATCH v5 01/12] iov_iter: Fix iov_iter_get_pages{,_alloc} page fault return value Andreas Gruenbacher
2021-08-03 19:18 ` [PATCH v5 02/12] powerpc/kvm: Fix kvm_use_magic_page Andreas Gruenbacher
2021-08-03 19:18 ` [PATCH v5 03/12] Turn fault_in_pages_{readable,writeable} into fault_in_{readable,writeable} Andreas Gruenbacher
2021-08-03 19:43   ` Linus Torvalds
2021-08-03 20:57   ` Al Viro
2021-08-03 21:38     ` Andreas Gruenbacher
2021-08-03 19:18 ` [PATCH v5 04/12] Turn iov_iter_fault_in_readable into fault_in_iov_iter_readable Andreas Gruenbacher
2021-08-03 19:18 ` [PATCH v5 05/12] iov_iter: Introduce fault_in_iov_iter_writeable Andreas Gruenbacher
2021-08-04 15:24   ` Andreas Gruenbacher
2021-08-03 19:18 ` [PATCH v5 06/12] gfs2: Add wrapper for iomap_file_buffered_write Andreas Gruenbacher
2021-08-03 19:18 ` Andreas Gruenbacher [this message]
2021-08-03 19:18 ` [PATCH v5 08/12] iomap: Fix iomap_dio_rw return value for user copies Andreas Gruenbacher
2021-08-03 19:18 ` [PATCH v5 09/12] iomap: Support restarting direct I/O requests after user copy failures Andreas Gruenbacher
2021-08-03 19:18 ` [PATCH v5 10/12] iomap: Add done_before argument to iomap_dio_rw Andreas Gruenbacher
2021-08-03 19:18 ` [PATCH v5 11/12] iov_iter: Introduce noio flag to disable page faults Andreas Gruenbacher
2021-08-03 19:18 ` [PATCH v5 12/12] gfs2: Fix mmap + page fault deadlocks for direct I/O Andreas Gruenbacher
2021-08-03 19:45 ` [PATCH v5 00/12] gfs2: Fix mmap + page fault deadlocks Linus Torvalds
2021-08-16 19:14   ` Andreas Gruenbacher
2021-08-18 21:49     ` Linus Torvalds
2021-08-19 19:40       ` Andreas Gruenbacher
2021-08-19 20:14         ` Linus Torvalds
2021-08-19 21:39           ` Andreas Gruenbacher

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