netdev - Re: [RFC PATCH 1/4] splice: Fix corruption of spliced data after splice() returns

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Date: Wed, 19 Jul 2023 17:02:04 -0400
From: Matt Whitlock <kernel@...twhitlock.name>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Matthew Wilcox <willy@...radead.org>,
 Miklos Szeredi <miklos@...redi.hu>,
 David Howells <dhowells@...hat.com>,
 <netdev@...r.kernel.org>,
 Dave Chinner <david@...morbit.com>,
 Jens Axboe <axboe@...nel.dk>,
 <linux-fsdevel@...ck.org>,
 <linux-mm@...ck.org>,
 <linux-kernel@...r.kernel.org>,
 Christoph Hellwig <hch@....de>,
 <linux-fsdevel@...r.kernel.org>
Subject: Re: [RFC PATCH 1/4] splice: Fix corruption of spliced data after splice() returns

On Wednesday, 19 July 2023 16:16:07 EDT, Linus Torvalds wrote:
> The *ONLY* reason for splice() existing is for zero-copy.

The very first sentence of splice(2) reads: "splice() moves data between 
two file descriptors without copying between kernel address space and user 
address space." Thus, it is not unreasonable to believe that the point of 
splice is to avoid copying between user-space and kernel-space.

If you use read() and write(), then you're making two copies. If you use 
splice(), then you're making one copy (or zero, but that's an optimization 
that should be invisible to the user).

> And no, we don't start some kind of crazy "versioned zero-copy with
> COW". That's a fundamental mistake.

Agreed. splice() should steal the reference if it can, copy the page data 
if it must. Note that, even in the slow case where the page data must be 
copied, this still gives a better-than-50% speedup over read()+write() 
since an entire copy (and one syscall) is elided.

> IF YOU DON'T UNDERSTAND THE *POINT* OF SPLICE, DON'T USE SPLICE.

Thanks for being so condescending. Your reputation is deserved.