[PATCH -next] f2fs: fix wrong dirty page count when race between mmap and fallocate.

[Shuqi Zhang]

This is a BUG_ON issue as follows when running xfstest-generic-503:
WARNING: CPU: 21 PID: 1385 at fs/f2fs/inode.c:762 f2fs_evict_inode+0x847/0xaa0
Modules linked in:
CPU: 21 PID: 1385 Comm: umount Not tainted 5.19.0-rc5+ #73
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-4.fc34 04/01/2014

Call Trace:
evict+0x129/0x2d0
dispose_list+0x4f/0xb0
evict_inodes+0x204/0x230
generic_shutdown_super+0x5b/0x1e0
kill_block_super+0x29/0x80
kill_f2fs_super+0xe6/0x140
deactivate_locked_super+0x44/0xc0
deactivate_super+0x79/0x90
cleanup_mnt+0x114/0x1a0
__cleanup_mnt+0x16/0x20
task_work_run+0x98/0x100
exit_to_user_mode_prepare+0x3d0/0x3e0
syscall_exit_to_user_mode+0x12/0x30
do_syscall_64+0x42/0x80
entry_SYSCALL_64_after_hwframe+0x46/0xb0

Function flow analysis when BUG occurs:
f2fs_fallocate                    mmap
                                  do_page_fault
                                    pte_spinlock  // ---lock_pte
                                    do_wp_page
                                      wp_page_shared
                                        pte_unmap_unlock   // unlock_pte
                                          do_page_mkwrite
                                          f2fs_vm_page_mkwrite
                                            down_read(i_mmap_sem)
                                            lock_page
                                            if (PageMappedToDisk(page))
                                              goto out;
                                            // set_page_dirty  --NOT RUN
                                            out: up_read(i_mmap_sem)
                                          lock_page
                                        finish_mkwrite_fault // unlock_pte
f2fs_collapse_range
  down_write(i_mmap_sem)
  truncate_pagecache
    unmap_mapping_pages
      i_mmap_lock_write // down_write(i_mmap_rwsem)
        ......
        zap_pte_range
          pte_offset_map_lock // ---lock_pte
          f2fs_set_data_page_dirty
            if (!PageDirty(page)) {
                                        fault_dirty_shared_page
                                          f2fs_set_data_page_dirty
                                            if (!PageDirty(page)) {
                                              __set_page_dirty_nobuffer
                                              f2fs_update_dirty_page // ++
                                            }
                                          unlock_page
              __set_page_dirty_nobuffers
              f2fs_update_dirty_page // page count++
            }
          pte_unmap_unlock  // --unlock_pte
      i_mmap_unlock_write  // up_write(i_mmap_rwsem)
  truncate_inode_pages
  up_write(i_mmap_sem)

When race happens between mmap-do_page_fault-wp_page_shared and
fallocate-truncate_pagecache-zap_pte_range, the zap_pte_range calls
function set_page_dirty without page lock. Besides, though
truncate_pagecache has immap and pte lock, wp_page_shared calls
fault_dirty_shared_page without any. In this case, two threads race
in f2fs_set_data_page_dirty function. Page is set to dirty only ONCE,
but the count is added TWICE by calling f2fs_update_dirty_page.
Thus the count of dirty page cannot accord with the real dirty pages.

Following is the solution to in case of race happens without any lock.
If making sure f2fs_vm_page_mkwrite calls set_page_dirty within immap
lock area, page will already be dirtied when running into
fault_dirty_shared_page-f2fs_set_data_page_dirty.
The count of dirty page will not be increased wrong times.

Signed-off-by: Shuqi Zhang <zhangshuqi3@xxxxxxxxxx>
---
 fs/f2fs/file.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index ce4905a073b3..d837359a9c00 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -140,7 +140,7 @@ static vm_fault_t f2fs_vm_page_mkwrite(struct vm_fault *vmf)
 	 * check to see if the page is mapped already (no holes)
 	 */
 	if (PageMappedToDisk(page))
-		goto out_sem;
+		goto set_dirty;
 
 	/* page is wholly or partially inside EOF */
 	if (((loff_t)(page->index + 1) << PAGE_SHIFT) >
@@ -150,14 +150,15 @@ static vm_fault_t f2fs_vm_page_mkwrite(struct vm_fault *vmf)
 		offset = i_size_read(inode) & ~PAGE_MASK;
 		zero_user_segment(page, offset, PAGE_SIZE);
 	}
-	set_page_dirty(page);
-	if (!PageUptodate(page))
-		SetPageUptodate(page);
 
 	f2fs_update_iostat(sbi, APP_MAPPED_IO, F2FS_BLKSIZE);
-	f2fs_update_time(sbi, REQ_TIME);
 
+set_dirty:
 	trace_f2fs_vm_page_mkwrite(page, DATA);
+	set_page_dirty(page);
+	if (!PageUptodate(page))
+		SetPageUptodate(page);
+	f2fs_update_time(sbi, REQ_TIME);
 out_sem:
 	filemap_invalidate_unlock_shared(inode->i_mapping);
 
-- 
2.25.1