linux-ext4 - Re: [PATCH] ext4: inline: do not convert when writing to memory map

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Message-ID: <20250520145708.GA432950@mit.edu>
Date: Tue, 20 May 2025 10:57:08 -0400
From: "Theodore Ts'o" <tytso@....edu>
To: Thadeu Lima de Souza Cascardo <cascardo@...lia.com>
Cc: Jan Kara <jack@...e.com>, Tao Ma <boyu.mt@...bao.com>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Eric Biggers <ebiggers@...gle.com>, linux-ext4@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-dev@...lia.com,
        syzbot+0c89d865531d053abb2d@...kaller.appspotmail.com,
        stable@...r.kernel.org
Subject: Re: [PATCH] ext4: inline: do not convert when writing to memory map

On Mon, May 19, 2025 at 07:42:46AM -0300, Thadeu Lima de Souza Cascardo wrote:
> inline data handling has a race between writing and writing to a memory
> map.
> 
> When ext4_page_mkwrite is called, it calls ext4_convert_inline_data, which
> destroys the inline data, but if block allocation fails, restores the
> inline data. In that process, we could have:
> 
> CPU1					CPU2
> destroy_inline_data
> 					write_begin (does not see inline data)
> restory_inline_data
> 					write_end (sees inline data)
> 
> The conversion inside ext4_page_mkwrite was introduced at commit
> 7b4cc9787fe3 ("ext4: evict inline data when writing to memory map"). This
> fixes a documented bug in the commit message, which suggests some
> alternatives fixes.

Your fix just reverts commit 7b4cc9787fe3, and removes the BUG_ON.
While this is great for shutting up the syzbot report, but it causes
file writes to an inline data file via a mmap to never get written
back to the storage device.  So you are replacing BUG_ON that can get
triggered on a race condition in case of a failed block allocation,
with silent data corruption.   This is not an improvement.

Thanks for trying to address this, but I'm not going to accept your
proposed fix.

     	    	 	       	       - Ted