linux-kernel - Re: [PATCH 04/11] ppc64/kexec_file: avoid stomping memory used by special regions

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Date:   Tue, 30 Jun 2020 11:40:59 +0530
From:   Hari Bathini <hbathini@...ux.ibm.com>
To:     piliu <piliu@...hat.com>, Michael Ellerman <mpe@...erman.id.au>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Kexec-ml <kexec@...ts.infradead.org>,
        Petr Tesarik <ptesarik@...e.cz>,
        Mahesh J Salgaonkar <mahesh@...ux.ibm.com>,
        Sourabh Jain <sourabhjain@...ux.ibm.com>,
        lkml <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...abs.org>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Thiago Jung Bauermann <bauerman@...ux.ibm.com>,
        Dave Young <dyoung@...hat.com>,
        Vivek Goyal <vgoyal@...hat.com>,
        Eric Biederman <ebiederm@...ssion.com>
Subject: Re: [PATCH 04/11] ppc64/kexec_file: avoid stomping memory used by
 special regions



On 30/06/20 9:00 am, piliu wrote:
> 
> 
> On 06/29/2020 01:55 PM, Hari Bathini wrote:
>>
>>
>> On 28/06/20 7:44 am, piliu wrote:
>>> Hi Hari,
>>
>> Hi Pingfan,
>>
>>>
>>> After a quick through for this series, I have a few question/comment on
>>> this patch for the time being. Pls see comment inline.
>>>
>>> On 06/27/2020 03:05 AM, Hari Bathini wrote:
>>>> crashkernel region could have an overlap with special memory regions
>>>> like  opal, rtas, tce-table & such. These regions are referred to as
>>>> exclude memory ranges. Setup this ranges during image probe in order
>>>> to avoid them while finding the buffer for different kdump segments.
>>
>> [...]
>>
>>>> +	/*
>>>> +	 * Use the locate_mem_hole logic in kexec_add_buffer() for regular
>>>> +	 * kexec_file_load syscall
>>>> +	 */
>>>> +	if (kbuf->image->type != KEXEC_TYPE_CRASH)
>>>> +		return 0;
>>> Can the ranges overlap [crashk_res.start, crashk_res.end]?  Otherwise
>>> there is no requirement for @exclude_ranges.
>>
>> The ranges like rtas, opal are loaded by f/w. They almost always overlap with
>> crashkernel region. So, @exclude_ranges is required to support kdump.
> f/w passes rtas/opal as service, then must f/w mark these ranges as
> fdt_reserved_mem in order to make kernel aware not to use these ranges?

It does. Actually, reserve_map + reserved-ranges are reserved as soon as
memblock allocator is ready but not before crashkernel reservation.
Check early_reserve_mem() call in kernel/prom.c

> Otherwise kernel memory allocation besides kdump can also overwrite
> these ranges.> 
> Hmm, revisiting reserve_crashkernel(). It seems not to take any reserved
> memory into consider except kernel text. Could it work based on memblock
> allocator?

So, kdump could possibly overwrite these regions which is why an exclude
range list is needed. Same thing was done in kexec-tools as well.

Thanks
Hari