linux-kernel - Re: [PATCH] fuse: dax: No-op writepages callback

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Message-ID: <6733b667ea3de_10bc6294d3@dwillia2-xfh.jf.intel.com.notmuch>
Date: Tue, 12 Nov 2024 12:11:20 -0800
From: Dan Williams <dan.j.williams@...el.com>
To: Asahi Lina <lina@...hilina.net>, Miklos Szeredi <miklos@...redi.hu>, "Dan
 Williams" <dan.j.williams@...el.com>
CC: Jan Kara <jack@...e.cz>, Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>, Matthew Wilcox <willy@...radead.org>,
	Sergio Lopez Pascual <slp@...hat.com>, <asahi@...ts.linux.dev>,
	<linux-fsdevel@...r.kernel.org>, <linux-kernel@...r.kernel.org>, Asahi Lina
	<lina@...hilina.net>
Subject: Re: [PATCH] fuse: dax: No-op writepages callback

Asahi Lina wrote:
> When using FUSE DAX with virtiofs, cache coherency is managed by the
> host. Disk persistence is handled via fsync() and friends, which are
> passed directly via the FUSE layer to the host. Therefore, there's no
> need to do dax_writeback_mapping_range(). All that ends up doing is a
> cache flush operation, which is not caught by KVM and doesn't do much,
> since the host and guest are already cache-coherent.
> 
> Since dax_writeback_mapping_range() checks that the inode block size is
> equal to PAGE_SIZE, this fixes a spurious WARN when virtiofs is used
> with a mismatched guest PAGE_SIZE and virtiofs backing FS block size
> (this happens, for example, when it's a tmpfs and the host and guest
> have a different PAGE_SIZE). FUSE DAX does not require any particular FS
> block size, since it always performs DAX mappings in aligned 2MiB
> blocks.
> 
> See discussion in [1].
> 
> [1] https://lore.kernel.org/lkml/20241101-dax-page-size-v1-1-eedbd0c6b08f@asahilina.net/T/#u
> 
> Suggested-by: Dan Williams <dan.j.williams@...el.com>
> Signed-off-by: Asahi Lina <lina@...hilina.net>
> ---
>  fs/fuse/dax.c | 7 ++-----
>  1 file changed, 2 insertions(+), 5 deletions(-)

Looks good to me, thanks for the discussion on this.

Acked-by: Dan Williams <dan.j.williams@...el.com>