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

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Message-ID: <36440c0f-8b8a-c401-684f-6f54ce9c061e@loongson.cn>
Date:   Tue, 14 Dec 2021 18:03:11 +0800
From:   Tiezhu Yang <yangtiezhu@...ngson.cn>
To:     Matthew Wilcox <willy@...radead.org>,
        David Laight <David.Laight@...lab.com>
Cc:     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>,
        Christophe Leroy <christophe.leroy@...roup.eu>
Subject: Re: [PATCH v2 0/2] kdump: simplify code

On 12/13/2021 10:43 PM, Matthew Wilcox wrote:
> 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.
>

Hi,

Thank you for your discussions.

The intention of this patchset is to simplify the related code with no
functional changes and no side effects.

At this moment, if you are OK, I will send v3 used with inline function
copy_to_user_or_kernel() to keep it simple, maybe other more changes can
be done in the future if no any side effect.

The v3 will contain the following three patches to make the changes
more clear:

kdump: vmcore: remove copy_to() and add copy_to_user_or_kernel()
kdump: crashdump: use copy_to_user_or_kernel() to simplify code
kdump: vmcore: crashdump: make variable type of userbuf as bool