linux-kernel - Re: [PATCH v6 3/5] vmcore: Introduce remap_oldmem_pfn

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Date:	Tue, 09 Jul 2013 14:49:48 +0900
From:	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>
To:	Vivek Goyal <vgoyal@...hat.com>
CC:	Michael Holzheu <holzheu@...ux.vnet.ibm.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	kexec@...ts.infradead.org,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Jan Willeke <willeke@...ibm.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 3/5] vmcore: Introduce remap_oldmem_pfn_range()

(2013/07/08 23:28), Vivek Goyal wrote:
> On Mon, Jul 08, 2013 at 11:28:39AM +0200, Michael Holzheu wrote:
>> On Mon, 08 Jul 2013 14:32:09 +0900
>> HATAYAMA Daisuke <d.hatayama@...fujitsu.com> wrote:
>>
>>> (2013/07/02 4:32), Michael Holzheu wrote:
>>>> For zfcpdump we can't map the HSA storage because it is only available
>>>> via a read interface. Therefore, for the new vmcore mmap feature we have
>>>> introduce a new mechanism to create mappings on demand.
>>>>
>>>> This patch introduces a new architecture function remap_oldmem_pfn_range()
>>>> that should be used to create mappings with remap_pfn_range() for oldmem
>>>> areas that can be directly mapped. For zfcpdump this is everything besides
>>>> of the HSA memory. For the areas that are not mapped by remap_oldmem_pfn_range()
>>>> a generic vmcore a new generic vmcore fault handler mmap_vmcore_fault()
>>>> is called.
>>>>
>>>
>>> This fault handler is only for s390 specific issue. Other architectures don't need
>>> this for the time being.
>>>
>>> Also, from the same reason, I'm doing this review based on source code only.
>>> I cannot run the fault handler on meaningful system, which is currently s390 only.
>>
>> You can test the code on other architectures if you do not map anything in advance.
>> For example you could just "return 0" in remap_oldmem_pfn_range():
>>
>> /*
>>   * Architectures may override this function to map oldmem
>>   */
>> int __weak remap_oldmem_pfn_range(struct vm_area_struct *vma,
>>                                    unsigned long from, unsigned long pfn,
>>                                    unsigned long size, pgprot_t prot)
>> {
>>          return 0;
>> }
>>
>> In that case for all pages the new mechanism would be used.
>>
>>>
>>> I'm also concerned about the fault handler covers a full range of vmcore, which
>>> could hide some kind of mmap() bug that results in page fault.
>>>
>>> So, the fault handler should be enclosed by ifdef CONFIG_S390 for the time being.
>>
>> I personally do not like that, but if Vivek and you prefer this, of course we
>> can do that.
>>
>> What about something like:
>>
>> #ifdef CONFIG_S390
>> static int mmap_vmcore_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
>> {
>> ...
>> }
>> #else
>> static int mmap_vmcore_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
>> {
>> 	BUG();
>> }
>> #endif
>
> I personally perfer not to special case it for s390 only and let the
> handler be generic.
>
> If there is a bug in remap_old_pfn_range(), only side affect is that
> we will fault in the page when it is accessed and that will be slow. BUG()
> sounds excessive. At max it could be WARN_ONCE().
>
> In regular cases for x86, this path should not even hit. So special casing
> it to detect issues with remap_old_pfn_range() does not sound very good
> to me. I would rather leave it as it is and if there are bugs and mmap()
> slows down, then somebody needs to debug it.
>

I agree to WARN_ONCE(). Then, we can notice bug at least if it occurs.

Interface is like this?

[generic]

bool __weak in_valid_fault_range(pgoff_t pgoff)
{
     return false;
}

[s390]

bool in_valid_fault_range(pgoff_t pgoff)
{
     loff_t offset = pgoff << PAGE_CACHE_SHIFT;
     u64 paddr = vmcore_offset_to_paddr(offset);

     return paddr < ZFCPDUMP_HSA_SIZE;
}

assuming vmcore_offset_to_paddr() that looks up vmcore_list and returns physical
address corresponding to given offset of vmcore. I guess this could return error
value if there's no entry corresponding to given offset in vmcore_list.

-- 
Thanks.
HATAYAMA, Daisuke

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