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From: Ming Lei <[email protected]>
To: Linus Torvalds <[email protected]>
Cc: Jens Axboe <[email protected]>,
	[email protected], [email protected],
	[email protected],
	Alexander Viro <[email protected]>,
	Stefan Hajnoczi <[email protected]>,
	Miklos Szeredi <[email protected]>,
	Bernd Schubert <[email protected]>,
	Nitesh Shetty <[email protected]>,
	Christoph Hellwig <[email protected]>,
	Ziyang Zhang <[email protected]>,
	[email protected]
Subject: Re: [PATCH 1/4] fs/splice: enhance direct pipe & splice for moving pages in kernel
Date: Tue, 14 Feb 2023 08:52:23 +0800	[thread overview]
Message-ID: <Y+rbR48vvhHDJlUF@T590> (raw)
In-Reply-To: <CAHk-=wgJsi7t7YYpuo6ewXGnHz2nmj67iWR6KPGoz5TBu34mWQ@mail.gmail.com>

On Mon, Feb 13, 2023 at 12:04:27PM -0800, Linus Torvalds wrote:
> On Sat, Feb 11, 2023 at 5:39 PM Ming Lei <[email protected]> wrote:
> >
> > >
> > >  (a) what's the point of MAY_READ? A non-readable page sounds insane
> > > and wrong. All sinks expect to be able to read.
> >
> > For example, it is one page which needs sink end to fill data, so
> > we needn't to zero it in source end every time, just for avoiding
> > leak kernel data if (unexpected)sink end simply tried to read from
> > the spliced page instead of writing data to page.
> 
> I still don't understand.
> 
> A sink *reads* the data. It doesn't write the data.
> 
> There's no point trying to deal with "if unexpectedly doing crazy
> things". If a sink writes the data, the sinkm is so unbelievably buggy
> that it's not even funny.
> 
> And having two flags that you then say "have to be used together" is pointless.

Actually I think it is fine to use the pipe buffer flags separately,
if MAY_READ/MAY_WRITE is set in source end, the sink side need to respect
it. All current in-tree source end actually implies both MAY_READ & MAY_WRITE.

> It's not two different flags if you can't use them separately!
> 
> So I think your explanations are anything *but* explaining what you
> want. They are just strange and not sensible.
> 
> Please explain to me in small words and simple sentences what it is
> you want. And no, if the explanation is "the sink wants to write to
> the buffer", then that's not an explanation, it's just insanity.
> 
> We *used* to have the concept of "gifting" the buffer explicitly to
> the sink, so that the sink could - instead of reading from it - decide
> to just use the whole buffer as-is long term. The idea was that tthe
> buffer woudl literally be moved from the source to the destination,
> ownership and all.
> 
> But if that's what you want, then it's not about "sink writes". It's
> literally about the splice() wanting to move not just the data, but
> the whole ownership of the buffer.

Yeah, it is actually transferring the buffer ownership, and looks
SPLICE_F_GIFT is exactly the case, but the driver side needs to set
QUEUE_FLAG_STABLE_WRITES for avoiding writeback to touch these pages.

Follows the idea:

file(devices(such as, fuse, ublk), produce pipe buffer) -> direct pipe -> file(consume the pipe buffer)

The 'consume' could be READ or WRITE.

So once SPLICE_F_GIFT is set from source side, the two buffer flags
aren't needed any more, right?

Please see the detailed explanation & use case in following link:

https://lore.kernel.org/linux-block/[email protected]/T/#m237e5973571b3d85df9fa519cf2c9762440009ba



Thanks,
Ming


  reply	other threads:[~2023-02-14  0:53 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-02-10 15:32 [PATCH 0/4] io_uring: add IORING_OP_READ[WRITE]_SPLICE_BUF Ming Lei
2023-02-10 15:32 ` [PATCH 1/4] fs/splice: enhance direct pipe & splice for moving pages in kernel Ming Lei
2023-02-11 15:42   ` Ming Lei
2023-02-11 18:57     ` Linus Torvalds
2023-02-12  1:39       ` Ming Lei
2023-02-13 20:04         ` Linus Torvalds
2023-02-14  0:52           ` Ming Lei [this message]
2023-02-14  2:35             ` Ming Lei
2023-02-14 11:03           ` Miklos Szeredi
2023-02-14 14:35             ` Ming Lei
2023-02-14 15:39               ` Miklos Szeredi
2023-02-15  0:11                 ` Ming Lei
2023-02-15 10:36                   ` Miklos Szeredi
2023-02-10 15:32 ` [PATCH 2/4] fs/splice: allow to ignore signal in __splice_from_pipe Ming Lei
2023-02-10 15:32 ` [PATCH 3/4] io_uring: add IORING_OP_READ[WRITE]_SPLICE_BUF Ming Lei
2023-02-11 15:45   ` Jens Axboe
2023-02-11 16:12     ` Ming Lei
2023-02-11 16:52       ` Jens Axboe
2023-02-12  3:22         ` Ming Lei
2023-02-12  3:55           ` Jens Axboe
2023-02-13  1:06             ` Ming Lei
2023-02-11 17:13   ` Jens Axboe
2023-02-12  1:48     ` Ming Lei
2023-02-12  2:42       ` Jens Axboe
2023-02-10 15:32 ` [PATCH 4/4] ublk_drv: support splice based read/write zero copy Ming Lei
2023-02-10 21:54 ` [PATCH 0/4] io_uring: add IORING_OP_READ[WRITE]_SPLICE_BUF Jens Axboe
2023-02-10 22:19   ` Jens Axboe
2023-02-11  5:13   ` Ming Lei
2023-02-11 15:45     ` Jens Axboe
2023-02-14 16:36 ` Stefan Hajnoczi

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