Message-ID: <CANubcdUZpf1DOVYnqjYBYCYB0J0D=3vk+efC-T98gpO=hPgD4A@mail.gmail.com>
Date: Thu, 29 Aug 2024 09:50:06 +0800
From: Stephen Zhang <starzhangzsd@...il.com>
To: Jan Kara <jack@...e.cz>
Cc: tytso@....edu, adilger.kernel@...ger.ca, jack@...e.com, 
	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org, 
	zhangshida@...inos.cn, Baolin Liu <liubaolin@...inos.cn>
Subject: Re: [PATCH 2/2] ext4: fix a potential assertion failure due to
 improperly dirtied buffer

Jan Kara <jack@...e.cz> 于2024年8月28日周三 19:45写道：
>
> On Fri 23-08-24 09:33:29, zhangshida wrote:
> > From: Shida Zhang <zhangshida@...inos.cn>
> >
> > On an old kernel version(4.19, ext3, data=journal, pagesize=64k),
> > an assertion failure will occasionally be triggered by the line below:
> > -----------
> > jbd2_journal_commit_transaction
> > {
> > ...
> > J_ASSERT_BH(bh, !buffer_dirty(bh));
> > /*
> > * The buffer on BJ_Forget list and not jbddirty means
> > ...
> > }
> > -----------
> >
> > The same condition may also be applied to the lattest kernel version.
> >
> > When blocksize < pagesize and we truncate a file, there can be buffers in
> > the mapping tail page beyond i_size. These buffers will be filed to
> > transaction's BJ_Forget list by ext4_journalled_invalidatepage() during
> > truncation. When the transaction doing truncate starts committing, we can
> > grow the file again. This calls __block_write_begin() which allocates new
> > blocks under these buffers in the tail page we go through the branch:
>                                              ^^ and we...
>
>
> >                         if (buffer_new(bh)) {
> >                                 clean_bdev_bh_alias(bh);
> >                                 if (folio_test_uptodate(folio)) {
> >                                         clear_buffer_new(bh);
> >                                         set_buffer_uptodate(bh);
> >                                         mark_buffer_dirty(bh);
> >                                         continue;
> >                                 }
> >                                 ...
> >                         }
> >
> > Hence buffers on BJ_Forget list of the committing transaction get marked
> > dirty and this triggers the jbd2 assertion.
> >
> > Teach ext4_block_write_begin() to properly handle files with data
> > journalling by avoiding dirtying them directly. Instead of
> > folio_zero_new_buffers() we use ext4_journalled_zero_new_buffers() which
> > takes care of handling journalling. We also don't need to mark new uptodate
> > buffers as dirty in ext4_block_write_begin(). That will be either done
> > either by block_commit_write() in case of success or by
> > folio_zero_new_buffers() in case of failure.
> >
> > Reported-by: Baolin Liu <liubaolin@...inos.cn>
> > Suggested-by: Jan Kara <jack@...e.cz>
> > Signed-off-by: Shida Zhang <zhangshida@...inos.cn>
>
> Looks mostly good. Just three small comments:
>
> > @@ -1083,11 +1090,11 @@ int ext4_block_write_begin(struct folio *folio, loff_t pos, unsigned len,
> >                       err = get_block(inode, block, bh, 1);
> >                       if (err)
> >                               break;
>
>
> > +                     if (should_journal_data)
> > +                             do_journal_get_write_access(handle, inode, bh);
>
> I'd move this inside the buffer_new() branch and add before it a comment:
>                         /*
>                          * We may be zeroing partial buffers or all new
>                          * buffers in case of failure. Prepare JBD2 for
>                          * that.
>                          */
>
> >                       if (buffer_new(bh)) {
> >                               if (folio_test_uptodate(folio)) {
> > -                                     clear_buffer_new(bh);
> >                                       set_buffer_uptodate(bh);
> > -                                     mark_buffer_dirty(bh);
>
> Here I'd add comment:
>                                 /*
>                                  * Unlike __block_write_begin() we leave
>                                  * dirtying of new uptodate buffers to
>                                  * ->write_end() time or
>                                  * folio_zero_new_buffers().
>                                  */
>
> > @@ -1117,7 +1124,11 @@ int ext4_block_write_begin(struct folio *folio, loff_t pos, unsigned len,
> >                       err = -EIO;
> >       }
> >       if (unlikely(err)) {
> > -             folio_zero_new_buffers(folio, from, to);
> > +             if (should_journal_data)
> > +                     ext4_journalled_zero_new_buffers(handle, inode, folio,
> > +                                                      from, to);
>
> I've realized there's a small bug in ext4_journalled_zero_new_buffers()
> that it calls write_end_fn() only if it zeroed a buffer. But for new
> uptodate buffers we also need to call write_end_fn() to persist the
> uptodate content (similarly as folio_zero_new_buffers() does it). So we
> need another preparatory patch moving write_end_fn() in
> ext4_journalled_zero_new_buffers() to be called also for uptodate pages.
>

Will do. And also thanks for the detailed explanation.

-Stephen

> > +             else
> > +                     folio_zero_new_buffers(folio, from, to);
> >       }
> >  #ifdef CONFIG_FS_ENCRYPTION
> >       else if (fscrypt_inode_uses_fs_layer_crypto(inode)) {
>
>                                                                 Honza
> --
> Jan Kara <jack@...e.com>
> SUSE Labs, CR

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