Re: [PATCH v2 -next] f2fs: fix wrong dirty page count when race between mmap and fallocate.

[Chao Yu]

On 2022/8/31 10:24, Shuqi Zhang wrote:
> 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(invalidate_lock)
>                                              lock_page
>                                              if (PageMappedToDisk(page))
>                                                goto out;
>                                              // set_page_dirty  --NOT RUN
>                                              out: up_read(invalidate_lock);
>                                          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
>             set_page_dirty
>              f2fs_dirty_data_folio
>                if (!folio_test_dirty(folio)) {
>                                          fault_dirty_shared_page
>                                            set_page_dirty
>                                              f2fs_dirty_data_folio
>                                                if (!folio_test_dirty(folio)) {
>                                                  filemap_dirty_folio
>                                                  f2fs_update_dirty_folio // ++
>                                                }
>                                              unlock_page
>                  filemap_dirty_folio
>                  f2fs_update_dirty_folio // page count++
>                }
>            pte_unmap_unlock  // --unlock_pte
>        i_mmap_unlock_write  // up_write(i_mmap_rwsem)
>    truncate_inode_pages
>    up_write(i_mmap_sem)

Will this race condition affact f2fs private field update?

Thanks,

> 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_dirty_data_folio function. Page is set to dirty only ONCE,
> but the count is added TWICE by calling filemap_dirty_folio.
> 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.
> Since folio_test_set_dirty in filemap_dirty_folio is atomic, judge return
> value will not be at risk of race.
>
> Signed-off-by: Shuqi Zhang <zhangshuqi3@xxxxxxxxxx>
> ---
>   fs/f2fs/checkpoint.c | 3 +--
>   fs/f2fs/data.c       | 3 +--
>   fs/f2fs/node.c       | 3 +--
>   3 files changed, 3 insertions(+), 6 deletions(-)
>
> diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
> index 8259e0fa97e1..3a3329bf1033 100644
> --- a/fs/f2fs/checkpoint.c
> +++ b/fs/f2fs/checkpoint.c
> @@ -448,8 +448,7 @@ static bool f2fs_dirty_meta_folio(struct address_space *mapping,
>   
>   	if (!folio_test_uptodate(folio))
>   		folio_mark_uptodate(folio);
> -	if (!folio_test_dirty(folio)) {
> -		filemap_dirty_folio(mapping, folio);
> +	if (filemap_dirty_folio(mapping, folio)) {
>   		inc_page_count(F2FS_M_SB(mapping), F2FS_DIRTY_META);
>   		set_page_private_reference(&folio->page);
>   		return true;
> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> index aa3ccddfa037..16c39bd948a0 100644
> --- a/fs/f2fs/data.c
> +++ b/fs/f2fs/data.c
> @@ -3697,8 +3697,7 @@ static bool f2fs_dirty_data_folio(struct address_space *mapping,
>   		folio_mark_uptodate(folio);
>   	BUG_ON(folio_test_swapcache(folio));
>   
> -	if (!folio_test_dirty(folio)) {
> -		filemap_dirty_folio(mapping, folio);
> +	if (filemap_dirty_folio(mapping, folio)) {
>   		f2fs_update_dirty_folio(inode, folio);
>   		return true;
>   	}
> diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c
> index e06a0c478b39..3506ffcb31f8 100644
> --- a/fs/f2fs/node.c
> +++ b/fs/f2fs/node.c
> @@ -2147,8 +2147,7 @@ static bool f2fs_dirty_node_folio(struct address_space *mapping,
>   	if (IS_INODE(&folio->page))
>   		f2fs_inode_chksum_set(F2FS_M_SB(mapping), &folio->page);
>   #endif
> -	if (!folio_test_dirty(folio)) {
> -		filemap_dirty_folio(mapping, folio);
> +	if (filemap_dirty_folio(mapping, folio)) {
>   		inc_page_count(F2FS_M_SB(mapping), F2FS_DIRTY_NODES);
>   		set_page_private_reference(&folio->page);
>   		return true;