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Date: Thu, 10 Jul 2014 13:25:51 -0400
From: Vivek Goyal <vgoyal@...hat.com>
To: Vitaly Kuznetsov <vkuznets@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Michael Holzheu <holzheu@...ux.vnet.ibm.com>,
HATAYAMA Daisuke <d.hatayama@...fujitsu.com>,
xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] mmap_vmcore: skip non-ram pages reported by
hypervisors
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@...hat.com>
> Reviewed-by: Andrew Jones <drjones@...hat.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
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