dax: Fix sleep in atomic contex in grab_mapping_entry()

Commit 642261ac995e: "dax: add struct iomap based DAX PMD support" has
introduced unmapping of page tables if huge page needs to be split in
grab_mapping_entry(). However the unmapping happens after
radix_tree_preload() call which disables preemption and thus
unmap_mapping_range() tries to acquire i_mmap_lock in atomic context
which is a bug. Fix the problem by moving unmapping before
radix_tree_preload() call.

Fixes: 642261ac995e01d7837db1f4b90181496f7e6835
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

1 files changed, 7 insertions, 8 deletions

diff --git a/fs/dax.c b/fs/dax.c
index ad131cd2605d..5bfd27b4a69c 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -333,14 +333,6 @@ restart:
                 }
 
                 spin_unlock_irq(&mapping->tree_lock);
-                err = radix_tree_preload(
-                                mapping_gfp_mask(mapping) & ~__GFP_HIGHMEM);
-                if (err) {
-                        if (pmd_downgrade)
-                                put_locked_mapping_entry(mapping, index, entry);
-                        return ERR_PTR(err);
-                }
-
                 /*
                  * Besides huge zero pages the only other thing that gets
                  * downgraded are empty entries which don't need to be
@@ -350,6 +342,13 @@ restart:
                         unmap_mapping_range(mapping,
                                 (index << PAGE_SHIFT) & PMD_MASK, PMD_SIZE, 0);
 
+                err = radix_tree_preload(
+                                mapping_gfp_mask(mapping) & ~__GFP_HIGHMEM);
+                if (err) {
+                        if (pmd_downgrade)
+                                put_locked_mapping_entry(mapping, index, entry);
+                        return ERR_PTR(err);
+                }
                 spin_lock_irq(&mapping->tree_lock);
 
                 if (pmd_downgrade) {