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Date:	Mon, 14 Jul 2014 14:47:46 +0200
From:	Vitaly Kuznetsov <vkuznets@...hat.com>
To:	"HATAYAMA\, Daisuke" <d.hatayama@...fujitsu.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Vivek Goyal <vgoyal@...hat.com>,
	Michael Holzheu <holzheu@...ux.vnet.ibm.com>,
	xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5] mmap_vmcore: skip non-ram pages reported by hypervisors

"HATAYAMA, Daisuke" <d.hatayama@...fujitsu.com> writes:

> (2014/07/14 18:16), Vitaly Kuznetsov wrote:
>> We have a special check in read_vmcore() handler to check if the page was
>> reported as ram or not by the hypervisor (pfn_is_ram()). However, when
>> vmcore is read with mmap() no such check is performed. That can lead to
>> unpredictable results, e.g. when running Xen PVHVM guest memcpy() after
>> mmap() on /proc/vmcore will hang processing HVMMEM_mmio_dm pages creating
>> enormous load in both DomU and Dom0.
>> 
>> Fix the issue by mapping each non-ram page to the zero page. Keep direct
>> path with remap_oldmem_pfn_range() to avoid looping through all pages on
>> bare metal.
>> 
>> The issue can also be solved by overriding remap_oldmem_pfn_range() in
>> xen-specific code, as remap_oldmem_pfn_range() was been designed for.
>> That, however, would involve non-obvious xen code path for all x86 builds
>> with CONFIG_XEN_PVHVM=y and would prevent all other hypervisor-specific
>> code on x86 arch from doing the same override.
>> 
>> Changes from v4:
>> - change map_size type size_t -> unsigned long
>> - use prot instead of vma->vm_page_prot inside remap_oldmem_pfn_checked()
>> 
>> Changes from v3:
>> - multi line comment style changes
>> - minor code style changes
>> 
>> Changes from v2:
>> - make remap_oldmem_pfn_checked() interface exactly match
>>    remap_oldmem_pfn_range()
>> - unmap mapped part inside remap_oldmem_pfn_checked() in case of failure so
>>    we don't need to take care of it in mmap_vmcore()
>> - create vmcore_remap_oldmem_pfn() wrapper
>> 
>> Changes from v1:
>> - comment style changes
>> - change remap_oldmem_pfn_checked() interface to closer match the
>>    remap_oldmem_pfn() interface
>> - preserve formal parameters within the loop, make the loop conditions
>>    easier to understand
>> - use my_zero_pfn() for the zero page
>> - return remapped length instead of new offset
>> 
>> Reviewed-by: Andrew Jones <drjones@...hat.com>
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>> ---
>>   fs/proc/vmcore.c | 83 ++++++++++++++++++++++++++++++++++++++++++++++++++++++--
>>   1 file changed, 80 insertions(+), 3 deletions(-)
>> 
>> diff --git a/fs/proc/vmcore.c b/fs/proc/vmcore.c
>> index 382aa89..1f77f35 100644
>> --- a/fs/proc/vmcore.c
>> +++ b/fs/proc/vmcore.c
>> @@ -328,6 +328,83 @@ static inline char *alloc_elfnotes_buf(size_t notes_sz)
>>    * virtually contiguous user-space in ELF layout.
>>    */
>>   #ifdef CONFIG_MMU
>> +/*
>> + * remap_oldmem_pfn_checked - do remap_oldmem_pfn_range replacing all pages
>> + * reported as not being ram with the zero page.
>> + *
>> + * @vma: vm_area_struct describing requested mapping
>> + * @from: start remapping from
>> + * @pfn: page frame number to start remapping to
>> + * @size: remapping size
>> + * @prot: protection bits
>> + *
>> + * Returns zero on success, -EAGAIN on failure.
>> + */
>> +int remap_oldmem_pfn_checked(struct vm_area_struct *vma, unsigned long from,
>> +			     unsigned long pfn, unsigned long size,
>> +			     pgprot_t prot)
>> +{
>> +	unsigned long map_size;
>> +	unsigned long pos_start, pos_end, pos;
>> +	unsigned long zeropage_pfn = my_zero_pfn(0);
>> +	u64 len = 0;
>
> Sorry, I missed this yesterday. 

Thanks for your review!

> This should also be fixed as size_t or unsigned long.
> Does 32-bit compiler warn about this at the call of do_munmap() below
> due to difference of bit length of the two types?

Mine doesn't. But you're right, it makes sense to make it match
do_munmap()'s interface and len there is size_t. I'll send v6 with this change.

>
>> +
>> +	pos_start = pfn;
>> +	pos_end = pfn + (size >> PAGE_SHIFT);
>> +
>> +	for (pos = pos_start; pos < pos_end; ++pos) {
>> +		if (!pfn_is_ram(pos)) {
>> +			/*
>> +			 * We hit a page which is not ram. Remap the continuous
>> +			 * region between pos_start and pos-1 and replace
>> +			 * the non-ram page at pos with the zero page.
>> +			 */
>> +			if (pos > pos_start) {
>> +				/* Remap continuous region */
>> +				map_size = (pos - pos_start) << PAGE_SHIFT;
>> +				if (remap_oldmem_pfn_range(vma, from + len,
>> +							   pos_start, map_size,
>> +							   prot))
>> +					goto fail;
>> +				len += map_size;
>> +			}
>> +			/* Remap the zero page */
>> +			if (remap_oldmem_pfn_range(vma, from + len,
>> +						   zeropage_pfn,
>> +						   PAGE_SIZE, prot))
>> +				goto fail;
>> +			len += PAGE_SIZE;
>> +			pos_start = pos + 1;
>> +		}
>> +	}
>> +	if (pos > pos_start) {
>> +		/* Remap the rest */
>> +		map_size = (pos - pos_start) << PAGE_SHIFT;
>> +		if (remap_oldmem_pfn_range(vma, from + len, pos_start,
>> +					   map_size, prot))
>> +			goto fail;
>> +		len += map_size;
>> +	}
>> +	return 0;
>> +fail:
>> +	do_munmap(vma->vm_mm, from, len);
>> +	return -EAGAIN;
>> +}
>> +
>> +int vmcore_remap_oldmem_pfn(struct vm_area_struct *vma,
>> +			    unsigned long from, unsigned long pfn,
>> +			    unsigned long size, pgprot_t prot)
>> +{
>> +	/*
>> +	 * Check if oldmem_pfn_is_ram was registered to avoid
>> +	 * looping over all pages without a reason.
>> +	 */
>> +	if (oldmem_pfn_is_ram)
>> +		return remap_oldmem_pfn_checked(vma, from, pfn, size, prot);
>> +	else
>> +		return remap_oldmem_pfn_range(vma, from, pfn, size, prot);
>> +}
>> +
>>   static int mmap_vmcore(struct file *file, struct vm_area_struct *vma)
>>   {
>>   	size_t size = vma->vm_end - vma->vm_start;
>> @@ -387,9 +464,9 @@ static int mmap_vmcore(struct file *file, struct vm_area_struct *vma)
>>   
>>   			tsz = min_t(size_t, m->offset + m->size - start, size);
>>   			paddr = m->paddr + start - m->offset;
>> -			if (remap_oldmem_pfn_range(vma, vma->vm_start + len,
>> -						   paddr >> PAGE_SHIFT, tsz,
>> -						   vma->vm_page_prot))
>> +			if (vmcore_remap_oldmem_pfn(vma, vma->vm_start + len,
>> +						    paddr >> PAGE_SHIFT, tsz,
>> +						    vma->vm_page_prot))
>>   				goto fail;
>>   			size -= tsz;
>>   			start += tsz;
>> 

-- 
  Vitaly
--
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