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Date: Wed, 13 May 2026 08:56:08 +0200
From: Christoph Hellwig <hch@lst.de>
To: "Darrick J. Wong" <djwong@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	Chris Li <chrisl@kernel.org>, Kairui Song <kasong@tencent.com>,
	Christian Brauner <brauner@kernel.org>,
	Jens Axboe <axboe@kernel.dk>, David Sterba <dsterba@suse.com>,
	Theodore Ts'o <tytso@mit.edu>, Jaegeuk Kim <jaegeuk@kernel.org>,
	Chao Yu <chao@kernel.org>, Trond Myklebust <trondmy@kernel.org>,
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	linux-cifs@vger.kernel.org
Subject: Re: [PATCH 08/12] swap,iomap: simplify iomap_swapfile_iter
Message-ID: <20260513065608.GA2250@lst.de>
References: <20260512053625.2950900-1-hch@lst.de> <20260512053625.2950900-9-hch@lst.de> <20260512170204.GI9555@frogsfrogsfrogs>
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User-Agent: Mutt/1.5.17 (2007-11-01)

On Tue, May 12, 2026 at 10:02:04AM -0700, Darrick J. Wong wrote:
> OH.  Now I remember why -- it's to handle contiguous mixed mappings
> better.
> 
> Let's say that you have a 1k fsblock filesystem and 4k base pages.  You
> fallocate an 8G swap file and then mkswap it.  The first mapping is a 1k
> written mapping at offset 0 for the swap header, followed by an 8388607k
> unwritten mapping at offset 3k.
> 
> The PAGE_SIZE rounding code in iomap_swapfile_add_extent will round the
> end of that first mapping down to zero and ignore it.  The second
> mapping will be treated as if it were a 8388604k mapping starting at
> offset 4096.  Now the page counts are wrong and the swapon fails.

Do we care about this use case?  I guess you did as you implemented
his, but still?

> 
> A more generic solution to this would be to change add_swap_extent to
> take sector_t addr and length values and use them to construct a bitmap
> representing contiguous physical space on the bdev, accounting of course
> for PAGE_SIZE alignment.  Except for the swap header page, every other
> contiguously set page-aligned region in the bitmap gets added to the
> swap extent map.

You don't even need a bitmap, just do basically the same checks as
the iomap code when moving to a new swap extent after moving to use
the sector_t.  And it really should anyway, as the current abuse of
sector_t to store a disk offset in PAGE_SIZE units is pretty gross.