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

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Message-ID: <51DBA029.3060009@jp.fujitsu.com>
Date:	Tue, 09 Jul 2013 14:31:21 +0900
From:	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>
To:	Michael Holzheu <holzheu@...ux.vnet.ibm.com>
CC:	Vivek Goyal <vgoyal@...hat.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 18:28), 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 meant without modifying source code at all. You say I need to define some function.

<cut>
>>
>>> +static int mmap_vmcore_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
>>> +{
>>> +	struct address_space *mapping = vma->vm_file->f_mapping;
>>> +	pgoff_t index = vmf->pgoff;
>>> +	struct page *page;
>>> +	loff_t src;
>>> +	char *buf;
>>> +	int rc;
>>> +
>>
>> You should check where faulting address points to valid range.
>> If the fault happens on invalid range, return VM_FAULT_SIGBUS.
>>
>> On s390 case, I think the range except for HSA should be thought of as invalid.
>>
>
> Hmmm, this would require another architecture backend interface. If we get a fault
> for a page outside of the HSA this would be a programming error in our code. Not
> sure if we should introduce new architecture interfaces just for sanity checks.
>

I think you need to introduce the backend interface since it's bug if it happens.
The current implementation hides such erroneous path due to generic implementation.

I also don't think it's big change from this version since you have already been
about to introduce several backend interfaces.

>>> +	page = find_or_create_page(mapping, index, GFP_KERNEL);
>>> +	if (!page)
>>> +		return VM_FAULT_OOM;
>>> +	if (!PageUptodate(page)) {
>>> +		src = index << PAGE_CACHE_SHIFT;
>>
>> src = (loff_t)index << PAGE_CACHE_SHIFT;
>>
>> loff_t has long long while index has unsigned long.
>
> Ok.
>
>> On s390 both might have the same byte length.
>
> On s390x both are 64 bit. On our 32 bit s390 archtecture long long is 64 bit
> and unsigned long 32 bit.
>
>> Also I prefer offset to src, but this is minor suggestion.
>
> Yes, I agree.
>
>>
>>> +		buf = (void *) (page_to_pfn(page) << PAGE_SHIFT);
>>
>> I found page_to_virt() macro.
>
> Looks like page_to_virt() is not usable on most architectures and probably
> pfn_to_kaddr(pfn) would be the correct thing here. Unfortunately is also not
> defined on s390.
>
> But when I discussed your comment with Martin, we found out that the current
>
> buf = (void *) (page_to_pfn(page) << PAGE_SHIFT);
>
> is not correct on x86. It should be:
>
> buf = __va((page_to_pfn(page) << PAGE_SHIFT));
>

It seems OK for this.

-- 
Thanks.
HATAYAMA, Daisuke

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