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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

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()). 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.

Could the 'remap_oldmem_pfn_range' become an function ops? I see there
is an 'register_oldmem_pfn_is_ram' - so could there be similar one for
'pfn_range'?

> 
> 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;
> +                             len += tsz;
> +                     }
>                       size -= tsz;
>                       start += tsz;
> -                     len += tsz;
>  
>                       if (size == 0)
>                               return 0;
> -- 
> 1.9.3
> 
> 
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