dax: improve comment about truncate race

Jan Kara pointed out I should be more explicit here about the perils of
racing against truncate.  The comment is mostly the same as for the PTE
case.

Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/dax.c b/fs/dax.c
index 2deed64..c694117 100644 (file)
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -553,7 +553,12 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
        if (!buffer_size_valid(&bh) || bh.b_size < PMD_SIZE)
                goto fallback;
 
-       /* Guard against a race with truncate */
+       /*
+        * If a truncate happened while we were allocating blocks, we may
+        * leave blocks allocated to the file that are beyond EOF.  We can't
+        * take i_mutex here, so just leave them hanging; they'll be freed
+        * when the file is deleted.
+        */
        size = (i_size_read(inode) + PAGE_SIZE - 1) >> PAGE_SHIFT;
        if (pgoff >= size) {
                result = VM_FAULT_SIGBUS;