[PATCH] dax: Fix sleep in atomic contex in grab_mapping_entry()

[Jan Kara]

Commit 7b5b8c9c4ac9 "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: 7b5b8c9c4ac9716fe9d77ec56ae5d962192ba030
Signed-off-by: Jan Kara <jack@xxxxxxx>
---
 fs/dax.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c
index 51b03e91d3e2..5c74f60d0a50 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -351,14 +351,6 @@ static void *grab_mapping_entry(struct address_space *mapping, pgoff_t index,
 		}
 
 		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
@@ -368,6 +360,13 @@ static void *grab_mapping_entry(struct address_space *mapping, pgoff_t index,
 			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) {
-- 
2.10.2


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