Re: [PATCH v5] mmap_vmcore: skip non-ram pages reported by hypervisors
From: Vitaly Kuznetsov
Date: Mon Jul 14 2014 - 08:48:10 EST
"HATAYAMA, Daisuke" <d.hatayama@xxxxxxxxxxxxxx> 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@xxxxxxxxxx>
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
>> ---
>> 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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/