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

[Jaegeuk Kim <jaegeuk@kernel.org>]

On 08/20, Chao Yu wrote:
> 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()?

Found this.

2637  * The caller must ensure this doesn't race with truncation.  Most will
2638  * simply hold the folio lock, but e.g. zap_pte_range() calls with the
2639  * folio mapped and the pte lock held, which also locks out truncation.
2640  */
2641 bool filemap_dirty_folio(struct address_space *mapping, struct folio *folio)
2642 {

> 
> 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@huawei.com>
> > ---
> >   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);


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