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

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Message-ID: <874jd5qqpq.fsf@metaspace.dk>
Date: Sun, 17 Mar 2024 08:09:53 +0100
From: Andreas Hindborg <nmi@...aspace.dk>
To: Matthew Wilcox <willy@...radead.org>
Cc: Jens Axboe <axboe@...nel.dk>,  Christoph Hellwig <hch@....de>,  Keith
 Busch <kbusch@...nel.org>,  Damien Le Moal <Damien.LeMoal@....com>,  Bart
 Van Assche <bvanassche@....org>,  Hannes Reinecke <hare@...e.de>,
  "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,  Andreas
 Hindborg <a.hindborg@...sung.com>,  Niklas Cassel <Niklas.Cassel@....com>,
  Greg KH <gregkh@...uxfoundation.org>,  Miguel Ojeda <ojeda@...nel.org>,
  Alex Gaynor <alex.gaynor@...il.com>,  Wedson Almeida Filho
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 González Collado <sergio.collado@...il.com>,  Joel
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 <kernel@...kajraghav.com>,  Daniel Gomez <da.gomez@...sung.com>,  open
 list <linux-kernel@...r.kernel.org>,  "rust-for-linux@...r.kernel.org"
 <rust-for-linux@...r.kernel.org>,  "lsf-pc@...ts.linux-foundation.org"
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Subject: Re: [RFC PATCH 0/5] Rust block device driver API and null block driver

Matthew Wilcox <willy@...radead.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