linux-ext4 - Re: [PATCH v2] ext4: fix race condition between buffer write and page

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Date:   Mon, 5 Jun 2023 15:55:35 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Jan Kara <jack@...e.cz>
Cc:     Theodore Ts'o <tytso@....edu>, Baokun Li <libaokun1@...wei.com>,
        linux-ext4@...r.kernel.org, adilger.kernel@...ger.ca,
        ritesh.list@...il.com, linux-kernel@...r.kernel.org,
        jun.nie@...aro.org, ebiggers@...nel.org, yi.zhang@...wei.com,
        yangerkun@...wei.com, yukuai3@...wei.com,
        syzbot+a158d886ca08a3fecca4@...kaller.appspotmail.com,
        stable@...r.kernel.org
Subject: Re: [PATCH v2] ext4: fix race condition between buffer write and
 page_mkwrite

On Mon, Jun 05, 2023 at 02:21:41PM +0200, Jan Kara wrote:
> On Mon 05-06-23 11:16:55, Jan Kara wrote:
> > Yeah, I agree, that is also the conclusion I have arrived at when thinking
> > about this problem now. We should be able to just remove the conversion
> > from ext4_page_mkwrite() and rely on write(2) or truncate(2) doing it when
> > growing i_size.
> 
> OK, thinking more about this and searching through the history, I've
> realized why the conversion is originally in ext4_page_mkwrite(). The
> problem is described in commit 7b4cc9787fe35b ("ext4: evict inline data
> when writing to memory map") but essentially it boils down to the fact that
> ext4 writeback code does not expect dirty page for a file with inline data
> because ext4_write_inline_data_end() should have copied the data into the
> inode and cleared the folio's dirty flag.
> 
> Indeed messing with xattrs from the writeback path to copy page contents
> into inline data xattr would be ... interesting. Hum, out of good ideas for
> now :-|.

Is it so bad?  Now that we don't have writepage in ext4, only
writepages, it seems like we have a considerably more benign locking
environment to work in.