linux-kernel - Re: [PATCH next 0/3] iov: Optimise user copies

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Message-ID: <20250322100851.39c9eb33@pumpkin>
Date: Sat, 22 Mar 2025 10:08:51 +0000
From: David Laight <david.laight.linux@...il.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>, David
 Howells <dhowells@...hat.com>, Matthew Wilcox <willy@...radead.org>, Andrew
 Morton <akpm@...ux-foundation.org>, Alexander Viro
 <viro@...iv.linux.org.uk>
Subject: Re: [PATCH next 0/3] iov: Optimise user copies

On Fri, 21 Mar 2025 16:35:52 -0700
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Fri, 21 Mar 2025 at 15:46, David Laight <david.laight.linux@...il.com> wrote:
> >
> > The speculation barrier in access_ok() is expensive.
> >
> > The first patch removes the initial checks when reading the iovec[].
> > The checks are repeated before the actual copy.
> >
> > The second patch uses 'user address masking' if supported.
> >
> > The third removes a lot of code for single entry iovec[].  
> 
> Ack, except I'd really like to see numbers for things that claim to
> remove expensive stuff.

Testing readv() of /dev/zero or writev() of /dev/null probably show
most gain.

I did do some allmodconfig builds and got no change, but I might have the
lfence compiled out of access_ok().
In any case kernel builds are pretty much user space limited.

	David

> 
> But yeah, the patches look sane.
> 
>           Linus