Re: [PATCH v3] mmap_vmcore: skip non-ram pages reported by hypervisors

[Vivek Goyal <vgoyal@redhat.com>]

On Thu, Jul 10, 2014 at 07:15:43PM +0200, 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 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
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> Reviewed-by: Andrew Jones <drjones@redhat.com>
> ---
>  fs/proc/vmcore.c | 82 +++++++++++++++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 79 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/proc/vmcore.c b/fs/proc/vmcore.c
> index 382aa89..66dac43 100644
> --- a/fs/proc/vmcore.c
> +++ b/fs/proc/vmcore.c
> @@ -328,6 +328,82 @@ 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)
> +{
> +	size_t map_size;
> +	unsigned long pos_start, pos_end, pos;
> +	unsigned long zeropage_pfn = my_zero_pfn(0);
> +	u64 len = 0;
> +
> +	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.
> +			 */

Hi Vitaly,

Thanks for the changes. Few more minor nits are below.

Can we use following comment style for multi line comments.

/*
 * multi line comment.
 */

> +			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))
						   ^^^^^
Do you still need multiple lines here to pass parameters. I think atleast
"prot" should right next to PAGE_SIZE. 

> +				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,
> +					   vma->vm_page_prot))
					   ^^^^^^^^^^^^^^
Same here. Looks like vma->vm_page_prot should fit in line above.

> +			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. */

Lets use multi line comment style.

Thanks
Vivek