linux-kernel - Re: [PATCH v2 0/2] kdump: simplify code

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Message-ID: <YbdcKK3Cq6ITTg/l@casper.infradead.org>
Date:   Mon, 13 Dec 2021 14:43:52 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     'Tiezhu Yang' <yangtiezhu@...ngson.cn>,
        Dave Young <dyoung@...hat.com>, Baoquan He <bhe@...hat.com>,
        Vivek Goyal <vgoyal@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-ia64@...r.kernel.org" <linux-ia64@...r.kernel.org>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "linux-sh@...r.kernel.org" <linux-sh@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Xuefeng Li <lixuefeng@...ngson.cn>
Subject: Re: [PATCH v2 0/2] kdump: simplify code

On Mon, Dec 13, 2021 at 08:30:33AM +0000, David Laight wrote:
> From: Matthew Wilcox
> > Sent: 12 December 2021 11:48
> > 
> > On Sat, Dec 11, 2021 at 05:53:46PM +0000, David Laight wrote:
> > > From: Tiezhu Yang
> > > > Sent: 11 December 2021 03:33
> > > >
> > > > v2:
> > > >   -- add copy_to_user_or_kernel() in lib/usercopy.c
> > > >   -- define userbuf as bool type
> > >
> > > Instead of having a flag to indicate whether the buffer is user or kernel,
> > > would it be better to have two separate buffer pointers.
> > > One for a user space buffer, the other for a kernel space buffer.
> > > Exactly one of the buffers should always be NULL.
> > 
> > No.  You should be using an iov_iter instead.  See
> > https://lore.kernel.org/all/Ya4bdB0UBJCZhUSo@casper.infradead.org/
> > for a start on this.
> 
> iov_iter gets horribly expensive...

Oh, right.  Reading the kcore is a high-performance path, my mistake.