linux-ext4 - Re: possible deadlock in start_this

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Date:   Sat, 13 Feb 2021 23:26:37 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Jan Kara <jack@...e.cz>
Cc:     jack@...e.com, linux-ext4@...r.kernel.org,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        tytso@....edu, mhocko@...e.cz, linux-mm@...ck.org,
        syzbot <syzbot+bfdded10ab7dcd7507ae@...kaller.appspotmail.com>
Subject: Re: possible deadlock in start_this_handle (2)

On 2021/02/11 19:49, Jan Kara wrote:
> This stacktrace should never happen. ext4_xattr_set() starts a transaction.
> That internally goes through start_this_handle() which calls:
> 
> 	handle->saved_alloc_context = memalloc_nofs_save();
> 
> and we restore the allocation context only in stop_this_handle() when
> stopping the handle. And with this fs_reclaim_acquire() should remove
> __GFP_FS from the mask and not call __fs_reclaim_acquire().

Excuse me, but it seems to me that nothing prevents ext4_xattr_set_handle() from reaching
ext4_xattr_inode_lookup_create() without memalloc_nofs_save() when hitting ext4_get_nojournal() path.
Will you explain when ext4_get_nojournal() path is executed?

ext4_xattr_set() {
  handle = ext4_journal_start(inode, EXT4_HT_XATTR, credits) == __ext4_journal_start() {
      return __ext4_journal_start_sb() {
        journal = EXT4_SB(sb)->s_journal;
        if (!journal || (EXT4_SB(sb)->s_mount_state & EXT4_FC_REPLAY))
          return ext4_get_nojournal(); // Never calls memalloc_nofs_save() despite returning !IS_ERR() value.
        return jbd2__journal_start(journal, blocks, rsv_blocks, revoke_creds, GFP_NOFS, type, line); // Calls memalloc_nofs_save() when start_this_handle() returns 0.
      }
    }
  }
  error = ext4_xattr_set_handle(handle, inode, name_index, name, value, value_len, flags); {
    ext4_write_lock_xattr(inode, &no_expand); // Grabs &ei->xattr_sem
    error = ext4_xattr_ibody_set(handle, inode, &i, &is) {
      error = ext4_xattr_set_entry(i, s, handle, inode, false /* is_block */) {
        ret = ext4_xattr_inode_lookup_create(handle, inode, i->value, i->value_len, &new_ea_inode); // Using GFP_KERNEL based on assumption that ext4_journal_start() called memalloc_nofs_save().
      }
    }
  }
}