Re: [RFC PATCH 0/5] Rust block device driver API and null block driver

From: Andreas Hindborg <nmi@metaspace.dk>
To: Matthew Wilcox <willy@infradead.org>
Cc: "Jens Axboe" <axboe@kernel.dk>, "Christoph Hellwig" <hch@lst.de>,
	"Keith Busch" <kbusch@kernel.org>,
	"Damien Le Moal" <Damien.LeMoal@wdc.com>,
	"Bart Van Assche" <bvanassche@acm.org>,
	"Hannes Reinecke" <hare@suse.de>,
	"linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
	"Andreas Hindborg" <a.hindborg@samsung.com>,
	"Niklas Cassel" <Niklas.Cassel@wdc.com>,
	"Greg KH" <gregkh@linuxfoundation.org>,
	"Miguel Ojeda" <ojeda@kernel.org>,
	"Alex Gaynor" <alex.gaynor@gmail.com>,
	"Wedson Almeida Filho" <wedsonaf@gmail.com>,
	"Boqun Feng" <boqun.feng@gmail.com>,
	"Gary Guo" <gary@garyguo.net>,
	"Björn Roy Baron" <bjorn3_gh@protonmail.com>,
	"Benno Lossin" <benno.lossin@proton.me>,
	"Alice Ryhl" <aliceryhl@google.com>,
	"Chaitanya Kulkarni" <chaitanyak@nvidia.com>,
	"Luis Chamberlain" <mcgrof@kernel.org>,
	"Yexuan Yang" <1182282462@bupt.edu.cn>,
	"Sergio González Collado" <sergio.collado@gmail.com>,
	"Joel Granados" <j.granados@samsung.com>,
	"Pankaj Raghav (Samsung)" <kernel@pankajraghav.com>,
	"Daniel Gomez" <da.gomez@samsung.com>,
	"open list" <linux-kernel@vger.kernel.org>,
	"rust-for-linux@vger.kernel.org" <rust-for-linux@vger.kernel.org>,
	"lsf-pc@lists.linux-foundation.org"
	<lsf-pc@lists.linux-foundation.org>,
	"gost.dev@samsung.com" <gost.dev@samsung.com>
Subject: Re: [RFC PATCH 0/5] Rust block device driver API and null block driver
Date: Sun, 17 Mar 2024 08:09:53 +0100	[thread overview]
Message-ID: <874jd5qqpq.fsf@metaspace.dk> (raw)
In-Reply-To: <ZfZajb_vcRwLacfH@casper.infradead.org> (Matthew Wilcox's message of "Sun, 17 Mar 2024 02:50:53 +0000")

Matthew Wilcox <willy@infradead.org> writes:

> On Wed, Mar 13, 2024 at 12:05:07PM +0100, Andreas Hindborg wrote:
>>  - Adopted the folio abstraction where applicable
>
> I don't think this was the correct move.  The memory you're managing
> with folios is only used for storing the data being stored in the blkdev.
> It's not mapped to userspace, it doesn't make use of the flags (locked,
> uptodate, writeback, etc), it doesn't need an LRU list, a mapping,
> an index, a refcount or memcg.
>
> I think you should use pages instead of folios to keep a handle on
> this memory.  Apologies for the confusion; we're in the middle of a
> years-long transition from the overly complex and overused struct page
> to splitting it into different data types for different purposes.

Ok, thanks. I'll swap it back.

I was under the impression that using folios also give the advantage
that handles are always head pages. No need to worry about head/tail
pages. If the driver moves to use higher order pages for larger block
sizes, would it then make sense with folios, or are page still
preferred?

> More detail on this here, if you're interested:
> https://kernelnewbies.org/MatthewWilcox/Memdescs

Thanks, that is useful.

Best regards,
Andreas