linux-kernel - Re: copy on write for splice() from file to pipe?

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Date:   Thu, 9 Feb 2023 11:36:02 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Stefan Metzmacher <metze@...ba.org>
Cc:     Jens Axboe <axboe@...nel.dk>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux API Mailing List <linux-api@...r.kernel.org>,
        io-uring <io-uring@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Samba Technical <samba-technical@...ts.samba.org>
Subject: Re: copy on write for splice() from file to pipe?

On Thu, Feb 9, 2023 at 11:17 AM Stefan Metzmacher <metze@...ba.org> wrote:
>
> Any comment about the idea of having a preadv2() flag that
> asks for a dma copy with something like async_memcpy() instead
> of the default that ends up in copy_user_enhanced_fast_string()?

I guarantee that you will only slow things down with some odd async_memcpy.

There are zero DMA engines that do memory copying better than memcpy -
the only way you can do better if is the destination isn't memory in
the first place but the final device, and then we just call it "dma".

                Linus