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

From: Jaegeuk Kim
Date: Sat Aug 20 2022 - 18:19:53 EST

On 08/20, Jaegeuk Kim wrote:
> 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@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);

Actually we don't need to call SetPageUptodate() since set_page_dirty() should
do? And, it seems the call stack is out-dated as well.

By the way, do we just need to get the right count by this?

--- 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;
}

> > > + f2fs_update_time(sbi, REQ_TIME);
> > > out_sem:
> > > filemap_invalidate_unlock_shared(inode->i_mapping);
>
>
> _______________________________________________
> Linux-f2fs-devel mailing list
> Linux-f2fs-devel@xxxxxxxxxxxxxxxxxxxxx
> https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel