linux-kernel - Re: [PATCH v6 05/15] ubifs: Rename whiteout atomically

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Date:   Mon, 10 Jan 2022 17:35:02 +0800
From:   Zhihao Cheng <chengzhihao1@...wei.com>
To:     Richard Weinberger <richard@....at>
CC:     Miquel Raynal <miquel.raynal@...tlin.com>,
        Vignesh Raghavendra <vigneshr@...com>,
        mcoquelin stm32 <mcoquelin.stm32@...il.com>,
        "kirill shutemov" <kirill.shutemov@...ux.intel.com>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        linux-mtd <linux-mtd@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 05/15] ubifs: Rename whiteout atomically

Hi, Richard
> 
> How do you make sure the the whiteout is never written to disk (by writeback) before ubifs_jnl_rename() linked
> it? That's the reason why other filesystems use the tmpfile mechanism for whiteouts too.
> 

The whiteout inode is clean after creation from create_whiteout(), and 
it can't be marked dirty until ubifs_jnl_rename() finished. So, I think 
there is no chance for whiteout being written on disk. Then, 
'ubifs_assert(c, !whiteout_ui->dirty)' never fails in ubifs_jnl_rename() 
during my local stress tests. You may add some delay executions after 
whiteout creation to make sure that whiteout won't be written back 
before ubifs_jnl_rename().

BTW, I considered plan B(Binding whiteout to a negative dentry), but we 
cannot get parent dir's entry in do_rename(), so I abandoned it.
--- a/fs/ubifs/dir.c
+++ b/fs/ubifs/dir.c
@@ -1334,7 +1335,8 @@ static int do_rename(struct inode *old_dir, struct 
dentry *old_dentry,
                         goto out_release;
                 }

-               err = do_tmpfile(old_dir, old_dentry, S_IFCHR | 
WHITEOUT_MODE, &whiteout);
+               // tmp_dentry = d_alloc(old_dir_dentry, &slash_name); 
// refer to vfs_tmpfile
+               err = do_tmpfile(old_dir, tmp_dentry, S_IFCHR | 
WHITEOUT_MODE, &whiteout);
                 if (err) {
                         kfree(dev);
                         goto out_release;