[PATCH v2] UBIFS: Fix assert failed in ubifs_set_page_dirty

[hujianyang]

Hi, all

Basing on the perious mail, I would like to show a clear figure
about the race I have found.

Thread A (mmap)                        Thread B (fsync)

->__do_fault                           ->write_cache_pages
   -> ubifs_page_mkwrite
       -> budget_space
       -> lock_page
       -> release/convert_page_budget
       -> SetPagePrivate
       -> TestSetPageDirty
       -> unlock_page
                                       -> lock_page
                                           -> TestClearPageDirty
                                           -> ubifs_writepage
                                               -> do_writepage
                                                   -> release_budget
                                                   -> ClearPagePrivate
                                                   -> unlock_page
   -> !(ret & VM_FAULT_LOCKED)
   -> lock_page
   -> set_page_dirty
       -> ubifs_set_page_dirty
           -> TestSetPageDirty (set page dirty without budgeting)
   -> unlock_page

According to this situation, my v2 fix returns from page_mkwrite
without performing unlock_page. We return VM_FAULT_LOCKED instead
of just return 0. After doing this, the race above will not happen.


Signed-off-by: hujianyang <hujianyang at huawei.com>
---
 fs/ubifs/file.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/fs/ubifs/file.c b/fs/ubifs/file.c
index 4f34dba..f7d48a0 100644
--- a/fs/ubifs/file.c
+++ b/fs/ubifs/file.c
@@ -1525,8 +1525,7 @@ static int ubifs_vm_page_mkwrite(struct vm_area_struct *vma,
 	}

 	wait_for_stable_page(page);
-	unlock_page(page);
-	return 0;
+	return VM_FAULT_LOCKED;

 out_unlock:
 	unlock_page(page);
-- 
1.8.5.5