[PATCH] nilfs2: Fix race condition that causes file system corruption

[Andreas Rohner <andreas.rohner-hi6Y0CQ0nG0@public.gmane.org>]

There is a race condition between the function nilfs_dirty_inode() and
nilfs_set_file_dirty().

When a file is opened, nilfs_dirty_inode() is called to update the
access timestamp in the inode. It calls __nilfs_mark_inode_dirty() in a
separate transaction. __nilfs_mark_inode_dirty() caches the ifile
buffer_head in the i_bh field of the inode info structure and marks it
as dirty.

After some data was written to the file in another transaction, the
function nilfs_set_file_dirty() is called, which adds the inode to
the ns_dirty_files list.

Then the segment construction calls nilfs_segctor_collect_dirty_files(),
which goes through the ns_dirty_files list and checks the i_bh field. If
there is a cached buffer_head in i_bh it is not marked as dirty again.

Since nilfs_dirty_inode() and nilfs_set_file_dirty() use separate
transactions, it is possible that a segment construction that
writes out the ifile occurs in-between the two. If this happens the
inode is not on the ns_dirty_files list, but its ifile block is still
marked as dirty and written out.

In the next segment construction, the data for the file is written out
and nilfs_bmap_propagate() updates the b-tree. Eventually the bmap root
is written into the i_bh block, which is not dirty, because it was
written out in another segment construction.

As a result the bmap update can be lost, which leads to file system
corruption. Either the virtual block address points to an unallocated
DAT block, or the DAT entry will be reused for something different.

The error can remain undetected for a long time. A typical error message
would be one of the "bad btree" errors or a warning that a DAT entry
could not be found.

This bug can be reproduced reliably by a simple benchmark that creates
and overwrites millions of 4k files.

Signed-off-by: Andreas Rohner <andreas.rohner-hi6Y0CQ0nG0@public.gmane.org>
---
 fs/nilfs2/segment.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/nilfs2/segment.c b/fs/nilfs2/segment.c
index 70ded52dc1dd..50e12956c737 100644
--- a/fs/nilfs2/segment.c
+++ b/fs/nilfs2/segment.c
@@ -1958,8 +1958,6 @@ static int nilfs_segctor_collect_dirty_files(struct nilfs_sc_info *sci,
 					  err, ii->vfs_inode.i_ino);
 				return err;
 			}
-			mark_buffer_dirty(ibh);
-			nilfs_mdt_mark_dirty(ifile);
 			spin_lock(&nilfs->ns_inode_lock);
 			if (likely(!ii->i_bh))
 				ii->i_bh = ibh;
@@ -1968,6 +1966,10 @@ static int nilfs_segctor_collect_dirty_files(struct nilfs_sc_info *sci,
 			goto retry;
 		}
 
+		// Always redirty the buffer to avoid race condition
+		mark_buffer_dirty(ii->i_bh);
+		nilfs_mdt_mark_dirty(ifile);
+
 		clear_bit(NILFS_I_QUEUED, &ii->i_state);
 		set_bit(NILFS_I_BUSY, &ii->i_state);
 		list_move_tail(&ii->i_dirty, &sci->sc_dirty_files);
-- 
2.13.2

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