From: Filipe Manana fdmanana@suse.com
commit 8184620ae21213d51eaf2e0bd4186baacb928172 upstream.
When doing a direct IO write using a iocb with nowait and dsync set, we end up not syncing the file once the write completes.
This is because we tell iomap to not call generic_write_sync(), which would result in calling btrfs_sync_file(), in order to avoid a deadlock since iomap can call it while we are holding the inode's lock and btrfs_sync_file() needs to acquire the inode's lock. The deadlock happens only if the write happens synchronously, when iomap_dio_rw() calls iomap_dio_complete() before it returns. Instead we do the sync ourselves at btrfs_do_write_iter().
For a nowait write however we can end up not doing the sync ourselves at at btrfs_do_write_iter() because the write could have been queued, and therefore we get -EIOCBQUEUED returned from iomap in such case. That makes us skip the sync call at btrfs_do_write_iter(), as we don't do it for any error returned from btrfs_direct_write(). We can't simply do the call even if -EIOCBQUEUED is returned, since that would block the task waiting for IO, both for the data since there are bios still in progress as well as potentially blocking when joining a log transaction and when syncing the log (writing log trees, super blocks, etc).
So let iomap do the sync call itself and in order to avoid deadlocks for the case of synchronous writes (without nowait), use __iomap_dio_rw() and have ourselves call iomap_dio_complete() after unlocking the inode.
A test case will later be sent for fstests, after this is fixed in Linus' tree.
Fixes: 51bd9563b678 ("btrfs: fix deadlock due to page faults during direct IO reads and writes") Reported-by: Марк Коренберг socketpair@gmail.com Link: https://lore.kernel.org/linux-btrfs/CAEmTpZGRKbzc16fWPvxbr6AfFsQoLmz-Lcg-7Og... CC: stable@vger.kernel.org # 6.0+ Signed-off-by: Filipe Manana fdmanana@suse.com Signed-off-by: David Sterba dsterba@suse.com ---
The commit in the Fixes tag was backported to 5.15 stable releases, so this patch is meant for 5.15.x and was tested on top of 5.15.77.
fs/btrfs/file.c | 39 ++++++++++++++++----------------------- 1 file changed, 16 insertions(+), 23 deletions(-)
diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c index 1c597cd6c024..90934711dcf0 100644 --- a/fs/btrfs/file.c +++ b/fs/btrfs/file.c @@ -1906,7 +1906,6 @@ static ssize_t check_direct_IO(struct btrfs_fs_info *fs_info,
static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from) { - const bool is_sync_write = (iocb->ki_flags & IOCB_DSYNC); struct file *file = iocb->ki_filp; struct inode *inode = file_inode(file); struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb); @@ -1917,6 +1916,7 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from) loff_t endbyte; ssize_t err; unsigned int ilock_flags = 0; + struct iomap_dio *dio;
if (iocb->ki_flags & IOCB_NOWAIT) ilock_flags |= BTRFS_ILOCK_TRY; @@ -1959,15 +1959,6 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from) goto buffered; }
- /* - * We remove IOCB_DSYNC so that we don't deadlock when iomap_dio_rw() - * calls generic_write_sync() (through iomap_dio_complete()), because - * that results in calling fsync (btrfs_sync_file()) which will try to - * lock the inode in exclusive/write mode. - */ - if (is_sync_write) - iocb->ki_flags &= ~IOCB_DSYNC; - /* * The iov_iter can be mapped to the same file range we are writing to. * If that's the case, then we will deadlock in the iomap code, because @@ -1986,12 +1977,23 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from) * So here we disable page faults in the iov_iter and then retry if we * got -EFAULT, faulting in the pages before the retry. */ -again: from->nofault = true; - err = iomap_dio_rw(iocb, from, &btrfs_dio_iomap_ops, &btrfs_dio_ops, - IOMAP_DIO_PARTIAL, written); + dio = __iomap_dio_rw(iocb, from, &btrfs_dio_iomap_ops, &btrfs_dio_ops, + IOMAP_DIO_PARTIAL, written); from->nofault = false;
+ /* + * iomap_dio_complete() will call btrfs_sync_file() if we have a dsync + * iocb, and that needs to lock the inode. So unlock it before calling + * iomap_dio_complete() to avoid a deadlock. + */ + btrfs_inode_unlock(inode, ilock_flags); + + if (IS_ERR_OR_NULL(dio)) + err = PTR_ERR_OR_ZERO(dio); + else + err = iomap_dio_complete(dio); + /* No increment (+=) because iomap returns a cumulative value. */ if (err > 0) written = err; @@ -2017,19 +2019,10 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from) } else { fault_in_iov_iter_readable(from, left); prev_left = left; - goto again; + goto relock; } }
- btrfs_inode_unlock(inode, ilock_flags); - - /* - * Add back IOCB_DSYNC. Our caller, btrfs_file_write_iter(), will do - * the fsync (call generic_write_sync()). - */ - if (is_sync_write) - iocb->ki_flags |= IOCB_DSYNC; - /* If 'err' is -ENOTBLK then it means we must fallback to buffered IO. */ if ((err < 0 && err != -ENOTBLK) || !iov_iter_count(from)) goto out;