Received-SPF: pass (google.com: domain of linux-ext4-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Mon, 5 Jun 2023 15:55:35 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Jan Kara <jack@suse.cz>
Cc:     Theodore Ts'o <tytso@mit.edu>, Baokun Li <libaokun1@huawei.com>,
        linux-ext4@vger.kernel.org, adilger.kernel@dilger.ca,
        ritesh.list@gmail.com, linux-kernel@vger.kernel.org,
        jun.nie@linaro.org, ebiggers@kernel.org, yi.zhang@huawei.com,
        yangerkun@huawei.com, yukuai3@huawei.com,
        syzbot+a158d886ca08a3fecca4@syzkaller.appspotmail.com,
        stable@vger.kernel.org
Subject: Re: [PATCH v2] ext4: fix race condition between buffer write and
 page_mkwrite
Message-ID: <ZH33ZzwyLFY48tfA@casper.infradead.org>
References: <20230530134405.322194-1-libaokun1@huawei.com>
 <20230604030445.GF1128744@mit.edu>
 <20230604210821.GA1257572@mit.edu>
 <ZH1BN+H1/Sa4eLQ4@casper.infradead.org>
 <20230605091655.24vl5fjesfskt3o5@quack3>
 <20230605122141.4njwwx3mrapqhvt4@quack3>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20230605122141.4njwwx3mrapqhvt4@quack3>
Precedence: bulk

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.