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Date:   Wed, 19 Jul 2023 22:21:04 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     "zhangpeng (AS)" <zhangpeng362@huawei.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, sidhartha.kumar@oracle.com,
        akpm@linux-foundation.org, wangkefeng.wang@huawei.com,
        sunnanyong@huawei.com
Subject: Re: [PATCH 1/6] mm/page_io: use a folio in __end_swap_bio_read()
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On Tue, Jul 18, 2023 at 05:16:15PM +0100, Matthew Wilcox wrote:
> How does _this_ folio end up in btrfs's write_dev_supers()?  This is a
> swap read.  The only folios which are swapped are anonymous and tmpfs.
> btrfs takes care of doing its own I/O.  wait_dev_supers() is looking
> for the error set in btrfs_end_super_write() which is the completion
> routine for write_dev_supers().  The pages involved there are attached
> to a btrfs address_space, not shmem or anon.

It actually operates on the block_device inode.  That does not matter
for this series, but complicates things in other ways.