linux-kernel - Re: [PATCH v8 00/17] gfs2: Fix mmap + page fault deadlocks

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Message-Id: <20211026094430.3669156-1-agruenba@redhat.com>
Date:   Tue, 26 Oct 2021 11:44:30 +0200
From:   Andreas Gruenbacher <agruenba@...hat.com>
To:     "Theodore Ts'o" <tytso@....edu>
Cc:     Andreas Gruenbacher <agruenba@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Paul Mackerras <paulus@...abs.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        "Darrick J. Wong" <djwong@...nel.org>, Jan Kara <jack@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        cluster-devel <cluster-devel@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        ocfs2-devel@....oracle.com, kvm-ppc@...r.kernel.org,
        linux-btrfs <linux-btrfs@...r.kernel.org>
Subject: Re: [PATCH v8 00/17] gfs2: Fix mmap + page fault deadlocks

Ted,

here's an updated version of Dave Hansen's original commit, but note
that generic/208 won't run on ext4 with data journaling enabled:

  $ MOUNT_OPTIONS='-o data=journal' TEST_DIR=/mnt/test TEST_DEV=/dev/vdb ./tests/generic/208
  QA output created by 208
  208 not run: ext4 data journaling doesn't support O_DIRECT

Thanks,
Andreas

--

Based on commit 998ef75ddb57 ("fs: do not prefault sys_write() user
buffer pages") by Dave Hansen <dave.hansen@...ux.intel.com>, but:

* Fix generic_perform_write as well as iomap_write_iter.

* copy_page_from_iter_atomic() doesn't trigger page faults, so there's no need
  to disable page faults around it [see commit 9e8c2af96e0d ("callers of
  iov_copy_from_user_atomic() don't need pagecache_disable()")].

* If fault_in_iov_iter_readable() fails to fault in the entire buffer,
  we still want to read everything up to the fault position.  This depends on
  commit a6294593e8a1 ("iov_iter: Turn iov_iter_fault_in_readable into
  fault_in_iov_iter_readable").

Signed-off-by: Andreas Gruenbacher <agruenba@...hat.com>
---
 fs/iomap/buffered-io.c | 20 +++++++-------------
 mm/filemap.c           | 20 +++++++-------------
 2 files changed, 14 insertions(+), 26 deletions(-)

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index 1753c26c8e76..d8809cd9ab31 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -744,17 +744,6 @@ static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
 		if (bytes > length)
 			bytes = length;
 
-		/*
-		 * Bring in the user page that we'll copy from _first_.
-		 * Otherwise there's a nasty deadlock on copying from the
-		 * same page as we're writing to, without it being marked
-		 * up-to-date.
-		 */
-		if (unlikely(fault_in_iov_iter_readable(i, bytes))) {
-			status = -EFAULT;
-			break;
-		}
-
 		status = iomap_write_begin(iter, pos, bytes, &page);
 		if (unlikely(status))
 			break;
@@ -777,9 +766,14 @@ static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
 			 * halfway through, might be a race with munmap,
 			 * might be severe memory pressure.
 			 */
-			if (copied)
+			if (copied) {
 				bytes = copied;
-			goto again;
+				goto again;
+			}
+			if (fault_in_iov_iter_readable(i, bytes) != bytes)
+				goto again;
+			status = -EFAULT;
+			break;
 		}
 		pos += status;
 		written += status;
diff --git a/mm/filemap.c b/mm/filemap.c
index 4dd5edcd39fd..467cdb7d086d 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3751,17 +3751,6 @@ ssize_t generic_perform_write(struct file *file,
 						iov_iter_count(i));
 
 again:
-		/*
-		 * Bring in the user page that we will copy from _first_.
-		 * Otherwise there's a nasty deadlock on copying from the
-		 * same page as we're writing to, without it being marked
-		 * up-to-date.
-		 */
-		if (unlikely(fault_in_iov_iter_readable(i, bytes))) {
-			status = -EFAULT;
-			break;
-		}
-
 		if (fatal_signal_pending(current)) {
 			status = -EINTR;
 			break;
@@ -3794,9 +3783,14 @@ ssize_t generic_perform_write(struct file *file,
 			 * halfway through, might be a race with munmap,
 			 * might be severe memory pressure.
 			 */
-			if (copied)
+			if (copied) {
 				bytes = copied;
-			goto again;
+				goto again;
+			}
+			if (fault_in_iov_iter_readable(i, bytes) != bytes)
+				goto again;
+			status = -EFAULT;
+			break;
 		}
 		pos += status;
 		written += status;
-- 
2.26.3