linux-kernel - Re: [ANNOUNCE] DAXFS: A zero-copy, dmabuf-friendly filesystem for shared memory

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Message-ID: <aXfRUlu61nrIqCZS@casper.infradead.org>
Date: Mon, 26 Jan 2026 20:40:50 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Cong Wang <cwang@...tikernel.io>
Cc: Gao Xiang <hsiangkao@...ux.alibaba.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, Cong Wang <xiyou.wangcong@...il.com>,
	multikernel@...ts.linux.dev
Subject: Re: [ANNOUNCE] DAXFS: A zero-copy, dmabuf-friendly filesystem for
 shared memory

On Mon, Jan 26, 2026 at 11:48:20AM -0800, Cong Wang wrote:
> Specifically for this scenario, struct inode is not compatible. This
> could rule out a lot of existing filesystems, except read-only ones.

I don't think you understand that there's a difference between *on disk*
inode and *in core* inode.  Compare and contrast struct ext2_inode and
struct inode.

> Now back to EROFS, it is still based on a block device, which
> itself can't be shared among different kernels. ramdax is actually
> a perfect example here, its label_area can't be shared among
> different kernels.
> 
> Let's take one step back: even if we really could share a device
> with multiple kernels, it still could not share the memory footprint,
> with DAX + EROFS, we would still get:
> 1) Each kernel creates its own DAX mappings
> 2) And faults pages independently
> 
> There is no cross-kernel page sharing accounting.
> 
> I hope this makes sense.

No, it doesn't.  I'm not suggesting that you use erofs unchanged, I'm
suggesting that you modify erofs to support your needs.