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Date: Wed, 28 Oct 2026 17:29:00 +0100
From: Christoph Hellwig <hch@lst.de>
To: Pavel Begunkov <asml.silence@gmail.com>
Cc: Anuj Gupta <anuj1072538@gmail.com>, hch@lst.de, djwong@kernel.org,
	cem@kernel.org, brauner@kernel.org, axboe@kernel.dk,
	linux-block@vger.kernel.org, linux-xfs@vger.kernel.org,
	linux-fsdevel@vger.kernel.org, joshi.k@samsung.com,
	anuj20.g@samsung.com, nj.shetty@samsung.com
Subject: Re: [RFC 0/4] fs: support dma-buf backed direct I/O
Message-ID: <20261028162900.GA22640@lst.de>
References: <20261016091919.971295-1-anuj1072538@gmail.com> <4649a20f-aba9-418e-987f-66f21ab24410@gmail.com>
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User-Agent: Mutt/1.5.17 (2007-11-01)

On Sun, Oct 18, 2026 at 11:18:27PM +0100, Pavel Begunkov wrote:
>  	file_ref_t			f_ref;
>  	/* --- cacheline 3 boundary (192 bytes) --- */
> +
> +	/*
> +	 * A tag representing the underlying subsystem for dma-buf I/O.
> +	 * See struct dma_buf_io_ctx::importer_tag.
> +	 */
> +	unsigned long			f_dma_io_tag;
>  } __randomize_layout

Growing strut file into a new cacheline just for dmabufs is a no-go..

Also note that a file can be placed on multiple block devices when
using RAID or just btrfs placements (not sure about f2fs).