f2fs: Fix a hungtask problem in atomic write

In the cache writing process, if it is an atomic file, increase the page
count of F2FS_WB_CP_DATA, otherwise increase the page count of
F2FS_WB_DATA.

When you step into the hook branch due to insufficient memory in
f2fs_write_begin, f2fs_drop_inmem_pages_all will be called to traverse
all atomic inodes and clear the FI_ATOMIC_FILE mark of all atomic files.

In f2fs_drop_inmem_pages，first acquire the inmem_lock , revoke all the
inmem_pages, and then clear the FI_ATOMIC_FILE mark. Before this mark is
cleared, other threads may hold inmem_lock to add inmem_pages to the inode
that has just been emptied inmem_pages, and increase the page count of
F2FS_WB_CP_DATA.

When the IO returns, it is found that the FI_ATOMIC_FILE flag is cleared
by f2fs_drop_inmem_pages_all, and f2fs_is_atomic_file returns false,which
causes the page count of F2FS_WB_DATA to be decremented. The page count of
F2FS_WB_CP_DATA cannot be cleared. Finally, hungtask is triggered in
f2fs_wait_on_all_pages because get_pages will never return zero.

process A:				process B:
f2fs_drop_inmem_pages_all
->f2fs_drop_inmem_pages of inode#1
    ->mutex_lock(&fi->inmem_lock)
    ->__revoke_inmem_pages of inode#1	f2fs_ioc_commit_atomic_write
    ->mutex_unlock(&fi->inmem_lock)	->f2fs_commit_inmem_pages of inode#1
					->mutex_lock(&fi->inmem_lock)
					->__f2fs_commit_inmem_pages
					    ->f2fs_do_write_data_page
					        ->f2fs_outplace_write_data
					            ->do_write_page
					                ->f2fs_submit_page_write
					                    ->inc_page_count(sbi, F2FS_WB_CP_DATA )
					->mutex_unlock(&fi->inmem_lock)
    ->spin_lock(&sbi->inode_lock[ATOMIC_FILE]);
    ->clear_inode_flag(inode, FI_ATOMIC_FILE)
    ->spin_unlock(&sbi->inode_lock[ATOMIC_FILE])
					f2fs_write_end_io
					->dec_page_count(sbi, F2FS_WB_DATA );

We can fix the problem by putting the action of clearing the FI_ATOMIC_FILE
mark into the inmem_lock lock. This operation can ensure that no one will
submit the inmem pages before the FI_ATOMIC_FILE mark is cleared, so that
there will be no atomic writes waiting for writeback.

Fixes: 57864ae5ce3a ("f2fs: limit # of inmemory pages")
Signed-off-by: Yi Zhuang <zhuangyi1@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
index 31ccea1..c517e68 100644 (file)
--- a/fs/f2fs/segment.c
+++ b/fs/f2fs/segment.c
@@ -324,23 +324,27 @@ void f2fs_drop_inmem_pages(struct inode *inode)
        struct f2fs_sb_info *sbi = F2FS_I_SB(inode);
        struct f2fs_inode_info *fi = F2FS_I(inode);
 
-       while (!list_empty(&fi->inmem_pages)) {
+       do {
                mutex_lock(&fi->inmem_lock);
+               if (list_empty(&fi->inmem_pages)) {
+                       fi->i_gc_failures[GC_FAILURE_ATOMIC] = 0;
+
+                       spin_lock(&sbi->inode_lock[ATOMIC_FILE]);
+                       if (!list_empty(&fi->inmem_ilist))
+                               list_del_init(&fi->inmem_ilist);
+                       if (f2fs_is_atomic_file(inode)) {
+                               clear_inode_flag(inode, FI_ATOMIC_FILE);
+                               sbi->atomic_files--;
+                       }
+                       spin_unlock(&sbi->inode_lock[ATOMIC_FILE]);
+
+                       mutex_unlock(&fi->inmem_lock);
+                       break;
+               }
                __revoke_inmem_pages(inode, &fi->inmem_pages,
                                                true, false, true);
                mutex_unlock(&fi->inmem_lock);
-       }
-
-       fi->i_gc_failures[GC_FAILURE_ATOMIC] = 0;
-
-       spin_lock(&sbi->inode_lock[ATOMIC_FILE]);
-       if (!list_empty(&fi->inmem_ilist))
-               list_del_init(&fi->inmem_ilist);
-       if (f2fs_is_atomic_file(inode)) {
-               clear_inode_flag(inode, FI_ATOMIC_FILE);
-               sbi->atomic_files--;
-       }
-       spin_unlock(&sbi->inode_lock[ATOMIC_FILE]);
+       } while (1);
 }
 
 void f2fs_drop_inmem_page(struct inode *inode, struct page *page)