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

[Vivek Goyal]

On Mon, Jul 07, 2014 at 05:05:49PM +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()).

I am wondering if this name pfn_is_ram() appropriate for what we are 
doing. So IIUC, a balooned memory is also RAM just that it has not 
been allocated yet. That means we can safely assume that there is no
data and can safely fill it with zeros?

If yes, then page_is_zero_filled() might be a more approprate name.

Also I am wondering why it was not done as part of copy_oldmem_page()
so that respective arch could hide all the details.

> 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.

I am not sure I understand this part. So what is "all other hypervisor
specic" code which will like to do this. And will that code is compiled
at the same time as CONFIG_XEN_PVHVM?

> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> ---
>  fs/proc/vmcore.c | 68 +++++++++++++++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 62 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/proc/vmcore.c b/fs/proc/vmcore.c
> index 382aa89..2716e19 100644
> --- a/fs/proc/vmcore.c
> +++ b/fs/proc/vmcore.c
> @@ -328,6 +328,46 @@ static inline char *alloc_elfnotes_buf(size_t notes_sz)
>   * virtually contiguous user-space in ELF layout.
>   */
>  #ifdef CONFIG_MMU
> +static u64 remap_oldmem_pfn_checked(struct vm_area_struct *vma, u64 len,
> +				    unsigned long pfn, unsigned long page_count)
> +{
> +	unsigned long pos;
> +	size_t size;
> +	unsigned long vma_addr;
> +	unsigned long emptypage_pfn = __pa(empty_zero_page) >> PAGE_SHIFT;
> +
> +	for (pos = pfn; (pos - pfn) <= page_count; pos++) {
> +		if (!pfn_is_ram(pos) || (pos - pfn) == page_count) {
> +			/* we hit a page which is not ram or reached the end */
> +			if (pos - pfn > 0) {
> +				/* remapping continuous region */
> +				size = (pos - pfn) << PAGE_SHIFT;
> +				vma_addr = vma->vm_start + len;
> +				if (remap_oldmem_pfn_range(vma, vma_addr,
> +							   pfn, size,
> +							   vma->vm_page_prot))
> +					return len;
> +				len += size;
> +				page_count -= (pos - pfn);
> +			}
> +			if (page_count > 0) {
> +				/* we hit a page which is not ram, replacing
> +				   with an empty one */
> +				vma_addr = vma->vm_start + len;
> +				if (remap_oldmem_pfn_range(vma, vma_addr,
> +							   emptypage_pfn,
> +							   PAGE_SIZE,
> +							   vma->vm_page_prot))
> +					return len;
> +				len += PAGE_SIZE;
> +				pfn = pos + 1;
> +				page_count--;
> +			}
> +		}
> +	}
> +	return len;
> +}
> +
>  static int mmap_vmcore(struct file *file, struct vm_area_struct *vma)
>  {
>  	size_t size = vma->vm_end - vma->vm_start;
> @@ -383,17 +423,33 @@ static int mmap_vmcore(struct file *file, struct vm_area_struct *vma)
>  
>  	list_for_each_entry(m, &vmcore_list, list) {
>  		if (start < m->offset + m->size) {
> -			u64 paddr = 0;
> +			u64 paddr = 0, original_len;
> +			unsigned long pfn, page_count;
>  
>  			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))
> -				goto fail;
> +
> +			/* check if oldmem_pfn_is_ram was registered to avoid
> +			   looping over all pages without a reason */
> +			if (oldmem_pfn_is_ram) {
> +				pfn = paddr >> PAGE_SHIFT;
> +				page_count = tsz >> PAGE_SHIFT;
> +				original_len = len;
> +				len = remap_oldmem_pfn_checked(vma, len, pfn,
> +							       page_count);
> +				if (len != original_len + tsz)
> +					goto fail;
> +			} else {
> +				if (remap_oldmem_pfn_range(vma,
> +							   vma->vm_start + len,
> +							   paddr >> PAGE_SHIFT,
> +							   tsz,
> +							   vma->vm_page_prot))
> +					goto fail;

Why are we defining both remap_oldmem_pfn_checked()? Can't we just
modify remap_oldmem_pfn_range() to *always* check for if
pfn_is_zero_filled() and map accordingly.

Thanks
Vivek
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