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Date:   Wed, 29 Jun 2022 10:40:08 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Qu Wenruo <quwenruo.btrfs@gmx.com>
Cc:     Chris Mason <clm@fb.com>, Christoph Hellwig <hch@lst.de>,
        Jan Kara <jack@suse.cz>, josef@toxicpanda.com,
        dsterba@suse.com, linux-btrfs@vger.kernel.org,
        linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH] btrfs: remove btrfs_writepage_cow_fixup
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References: <20220624122334.80603-1-hch@lst.de> <7c30b6a4-e628-baea-be83-6557750f995a@gmx.com> <20220624125118.GA789@lst.de> <20220624130750.cu26nnm6hjrru4zd@quack3.lan> <20220625091143.GA23118@lst.de> <c4af4c49-c537-bd6d-c27e-fe9ed71b9a8e@fb.com> <b29ee79c-e0d9-7ebe-a563-ca7f33130fc9@gmx.com>
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On Wed, Jun 29, 2022 at 09:20:59AM +0800, Qu Wenruo wrote:
> In fact, COW is not that special, even before btrfs or all the other
> fses supporting COW, all those old fses has to do something like COW,
> when they are writing into holes.
>
> What makes btrfs special is its csum, and in fact csum requires more
> stable page status.
>
> If someone can modify a page without waiting for its writeback to
> finish, btrfs csum can easily be stale and cause -EIO for future read.

And the writepage time fixup does not help with this at all, as it
just allocates the ordered extent at writepage time, long after the
data has been modified.