Re: [Xen-devel] [PATCH v2] mmap_vmcore: skip non-ram pages reported by hypervisors

[Vivek Goyal <vgoyal@xxxxxxxxxx>]

On Wed, Jul 09, 2014 at 01:40:22PM +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 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@xxxxxxxxxx>
> Reviewed-by: Andrew Jones <drjones@xxxxxxxxxx>

Also please cc HATAYAMA Daisuke <d.hatayama@xxxxxxxxxxxxxx> in next
posting. He did the mmap() work and might be interested in having a
look at the patch.

Thanks
Vivek

> ---
>  fs/proc/vmcore.c | 89 
> ++++++++++++++++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 84 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/proc/vmcore.c b/fs/proc/vmcore.c
> index 382aa89..5cd13f8 100644
> --- a/fs/proc/vmcore.c
> +++ b/fs/proc/vmcore.c
> @@ -328,6 +328,67 @@ 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 replacing all pages 
> reported
> + * as not being ram with the zero page.
> + *
> + * @vma: vm_area_struct describing requested mapping
> + * @vma_addr: start remapping from
> + * @pfn: page frame number to start remapping to
> + * @size: remapping size
> + *
> + * Returns the remapped length. If no errors were hit during the remapping it
> + * should be equal to size.
> + */
> +static u64 remap_oldmem_pfn_checked(struct vm_area_struct *vma,
> +                                 unsigned long vma_addr, unsigned long pfn,
> +                                 unsigned long size)
> +{
> +     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.
> +                      */
> +                     if (pos > pos_start) {
> +                             /* Remap continuous region */
> +                             map_size = (pos - pos_start) << PAGE_SHIFT;
> +                             if (remap_oldmem_pfn_range(vma, vma_addr + len,
> +                                                        pos_start, map_size,
> +                                                        vma->vm_page_prot))
> +                                     return len;
> +                             len += map_size;
> +                     }
> +                     /* Remap the zero page */
> +                     if (remap_oldmem_pfn_range(vma, vma_addr + len,
> +                                                zeropage_pfn,
> +                                                PAGE_SIZE,
> +                                                vma->vm_page_prot))
> +                             return len;
> +                     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, vma_addr + len, pos_start,
> +                                        map_size,
> +                                        vma->vm_page_prot))
> +                     return len;
> +             len += map_size;
> +     }
> +     return len;
> +}
> +
>  static int mmap_vmcore(struct file *file, struct vm_area_struct *vma)
>  {
>       size_t size = vma->vm_end - vma->vm_start;
> @@ -387,13 +448,31 @@ 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))
> -                             goto fail;
> +
> +                     /* Check if oldmem_pfn_is_ram was registered to avoid
> +                        looping over all pages without a reason. */
> +                     if (oldmem_pfn_is_ram) {
> +                             u64 original_len;
> +                             unsigned long pfn, vma_addr;
> +
> +                             pfn = paddr >> PAGE_SHIFT;
> +                             vma_addr = vma->vm_start + len;
> +                             original_len = len;
> +                             len += remap_oldmem_pfn_checked(vma, vma_addr,
> +                                                             pfn, tsz);
> +                             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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