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

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Message-ID: <Y+T/GE77AKzsPte9@casper.infradead.org>
Date:   Thu, 9 Feb 2023 14:11:36 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Stefan Metzmacher <metze@...ba.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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 09, 2023 at 02:55:59PM +0100, Stefan Metzmacher wrote:
> Hi Linus and others,
> 
> as written in a private mail before, I'm currently trying to
> make use of IORING_OP_SPLICE in order to get zero copy support
> in Samba.

I have to ask why.  In a modern network, isn't all data encrypted?
So you have to encrypt into a different buffer, and then you checksum
that buffer.  So it doesn't matter if writes can change the page cache
after you called splice(), you just need to have the data be consistent
so the checksum doesn't change.