When a file is opened with O_TRUNC, we need to make sure that any other
DAX operation is not in progress. DAX expects i_size to be stable.

In fuse_iomap_begin() we check for i_size at multiple places and we expect
i_size to not change.

Another problem is, if we setup a mapping in fuse_iomap_begin(), and
file gets truncated and dax read/write happens, KVM currently hangs.
It tries to fault in a page which does not exist on host (file got
truncated). It probably requries fixing in KVM.

So for now, take inode lock. Once KVM is fixed, we might have to
have a look at it again.

Signed-off-by: Vivek Goyal <vgo...@redhat.com>
---
 fs/fuse/file.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 9b82d9b4ebc3..d0979dc32f08 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -420,7 +420,7 @@ int fuse_open_common(struct inode *inode, struct file 
*file, bool isdir)
        int err;
        bool lock_inode = (file->f_flags & O_TRUNC) &&
                          fc->atomic_o_trunc &&
-                         fc->writeback_cache;
+                         (fc->writeback_cache || IS_DAX(inode));
 
        err = generic_file_open(inode, file);
        if (err)
-- 
2.20.1

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