Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::3:1 as permitted sender) client-ip=2620:137:e000::3:1;
Date:   Mon, 11 Dec 2023 23:35:31 -0800
From:   Christoph Hellwig <hch@infradead.org>
To:     Gao Xiang <hsiangkao@linux.alibaba.com>
Cc:     Eric Curtin <ecurtin@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-unionfs@vger.kernel.org, linux-erofs@lists.ozlabs.org,
        Daan De Meyer <daan.j.demeyer@gmail.com>,
        Stephen Smoogen <ssmoogen@redhat.com>,
        Yariv Rachmani <yrachman@redhat.com>,
        Daniel Walsh <dwalsh@redhat.com>,
        Douglas Landgraf <dlandgra@redhat.com>,
        Alexander Larsson <alexl@redhat.com>,
        Colin Walters <walters@redhat.com>,
        Brian Masney <bmasney@redhat.com>,
        Eric Chanudet <echanude@redhat.com>,
        Pavol Brilla <pbrilla@redhat.com>,
        Lokesh Mandvekar <lmandvek@redhat.com>,
        Petr =?utf-8?Q?=C5=A0abata?= <psabata@redhat.com>,
        Lennart Poettering <lennart@poettering.net>,
        Luca Boccassi <bluca@debian.org>, Neal Gompa <neal@gompa.dev>,
        nvdimm@lists.linux.dev
Subject: Re: [RFC KERNEL] initoverlayfs - a scalable initial filesystem
Message-ID: <ZXgNQ85PdUKrQU1j@infradead.org>
References: <CAOgh=Fwb+JCTQ-iqzjq8st9qbvauxc4gqqafjWG2Xc08MeBabQ@mail.gmail.com>
 <941aff31-6aa4-4c37-bb94-547c46250304@linux.alibaba.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <941aff31-6aa4-4c37-bb94-547c46250304@linux.alibaba.com>
Precedence: bulk

On Tue, Dec 12, 2023 at 08:50:56AM +0800, Gao Xiang wrote:
> For non-virtualization cases, I guess you could try to use `memmap`
> kernel option [2] to specify a memory region by bootloaders which
> contains an EROFS rootfs and a customized init for booting as
> erofs+overlayfs at least for `initoverlayfs`.  The main benefit is
> that the memory region specified by the bootloader can be directly
> used for mounting.  But I never tried if this option actually works.
> 
> Furthermore, compared to traditional ramdisks, using direct address
> can avoid page cache totally for uncompressed files like it can
> just use unencoded data as mmaped memory.  For compressed files, it
> still needs page cache to support mmaped access but we could adapt
> more for persistent memory scenarios such as disable cache
> decompression compared to previous block devices.
> 
> I'm not sure if it's worth implementing this in kernelspace since
> it's out of scope of an individual filesystem anyway.

IFF the use case turns out to be generally useful (it looks quite
convoluted and odd to me), we could esily do an initdax concept where
a chunk of memory passed by the bootloader is presented as a DAX device
properly without memmap hacks.