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

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Message-ID: <CAJfpeguawgi_Hnn2BwieNntbOCB1ghyijEtUOh4QyOrPis--dw@mail.gmail.com>
Date: Wed, 13 Nov 2024 11:48:47 +0100
From: Miklos Szeredi <miklos@...redi.hu>
To: Asahi Lina <lina@...hilina.net>
Cc: Dan Williams <dan.j.williams@...el.com>, 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, 
	Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [PATCH] fuse: dax: No-op writepages callback

On Tue, 12 Nov 2024 at 20:55, Asahi Lina <lina@...hilina.net> 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.

The conclusion seems convincing.  But adding Vivek, who originally
added this in commit 9483e7d5809a ("virtiofs: define dax address space
operations").

What I'm not clearly seeing is how virtually aliased CPU caches
interact with this.  In mm/filemap.c I see the flush_dcache_folio()
calls which deal with the kernel mapping of a page being in a
different cacheline as the user mapping.  How does that work in the
virt environment?

Also I suggest to remove the writepages callback instead of leaving it
as a no-op.

Thanks,
Miklos