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

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Message-ID: <6e96ae5a-91fd-726e-1eda-314f2317d8b4@linux.ibm.com>
Date:   Wed, 1 Jul 2020 23:48:03 +0530
From:   Hari Bathini <hbathini@...ux.ibm.com>
To:     Dave Young <dyoung@...hat.com>
Cc:     Thiago Jung Bauermann <bauerman@...ux.ibm.com>,
        Pingfan Liu <piliu@...hat.com>,
        Kexec-ml <kexec@...ts.infradead.org>,
        Mahesh J Salgaonkar <mahesh@...ux.ibm.com>,
        Petr Tesarik <ptesarik@...e.cz>,
        lkml <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...abs.org>,
        Sourabh Jain <sourabhjain@...ux.ibm.com>,
        Vivek Goyal <vgoyal@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Eric Biederman <ebiederm@...ssion.com>
Subject: Re: [PATCH 04/11] ppc64/kexec_file: avoid stomping memory used by
 special regions



On 01/07/20 1:10 pm, Dave Young wrote:
> Hi Hari,
> On 06/27/20 at 12:35am, 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.
>> Implement kexec_locate_mem_hole_ppc64() that locates a memory hole
>> accounting for these ranges. Also, override arch_kexec_add_buffer()
>> to locate a memory hole & later call __kexec_add_buffer() function
>> with kbuf->mem set to skip the generic locate memory hole lookup.
>>
>> Signed-off-by: Hari Bathini <hbathini@...ux.ibm.com>
>> ---
>>  arch/powerpc/include/asm/crashdump-ppc64.h |   10 +
>>  arch/powerpc/include/asm/kexec.h           |    7 -
>>  arch/powerpc/kexec/elf_64.c                |    7 +
>>  arch/powerpc/kexec/file_load_64.c          |  292 ++++++++++++++++++++++++++++
>>  4 files changed, 312 insertions(+), 4 deletions(-)
>>  create mode 100644 arch/powerpc/include/asm/crashdump-ppc64.h
>>
> [snip]
>>  /**
>> + * get_exclude_memory_ranges - Get exclude memory ranges. This list includes
>> + *                             regions like opal/rtas, tce-table, initrd,
>> + *                             kernel, htab which should be avoided while
>> + *                             setting up kexec load segments.
>> + * @mem_ranges:                Range list to add the memory ranges to.
>> + *
>> + * Returns 0 on success, negative errno on error.
>> + */
>> +static int get_exclude_memory_ranges(struct crash_mem **mem_ranges)
>> +{
>> +	int ret;
>> +
>> +	ret = add_tce_mem_ranges(mem_ranges);
>> +	if (ret)
>> +		goto out;
>> +
>> +	ret = add_initrd_mem_range(mem_ranges);
>> +	if (ret)
>> +		goto out;
>> +
>> +	ret = add_htab_mem_range(mem_ranges);
>> +	if (ret)
>> +		goto out;
>> +
>> +	ret = add_kernel_mem_range(mem_ranges);
>> +	if (ret)
>> +		goto out;
>> +
>> +	ret = add_rtas_mem_range(mem_ranges, false);
>> +	if (ret)
>> +		goto out;
>> +
>> +	ret = add_opal_mem_range(mem_ranges, false);
>> +	if (ret)
>> +		goto out;
>> +
>> +	ret = add_reserved_ranges(mem_ranges);
>> +	if (ret)
>> +		goto out;
>> +
>> +	/* exclude memory ranges should be sorted for easy lookup */
>> +	sort_memory_ranges(*mem_ranges);
>> +out:
>> +	if (ret)
>> +		pr_err("Failed to setup exclude memory ranges\n");
>> +	return ret;
>> +}
> 
> I'm confused about the "overlap with crashkernel memory", does that mean
> those normal kernel used memory could be put in crashkernel reserved

There are regions that could overlap with crashkernel region but they are
not normal kernel used memory though. These are regions that kernel and/or
f/w chose to place at a particular address for real mode accessibility
and/or memory layout between kernel & f/w kind of thing.

> memory range?  If so why can't just skip those areas while crashkernel
> doing the reservation?

crashkernel region has a dependency to be in the first memory block for it
to be accessible in real mode. Accommodating this requirement while addressing
other requirements would mean something like what we have now. A list of
possible special memory regions in crashkernel region to take care of.

I have plans to split crashkernel region into low & high to have exclusive
regions for crashkernel, even if that means to have two of them. But that
is for another day with its own set of complexities to deal with...

Thanks
Hari