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

Andreas Gruenbacher
2021-10-26T09:45:06Z

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@linux.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@redhat.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