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

[Chao Yu]

On 2022/8/15 16:15, 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(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

I didn't get it, why zap_pte_range() can set page dirty w/o lock_page?

I found it's very easy to reproduce this bug, but previously I never saw this...
is there any code udpate around truncate_pagecache()?

Thanks,

>              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);
>