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Date: Wed, 09 Jul 2014 11:46:40 +0200
From: Vitaly Kuznetsov <vkuznets@...hat.com>
To: Vivek Goyal <vgoyal@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Michael Holzheu <holzheu@...ux.vnet.ibm.com>,
xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
Olaf Hering <olaf@...fle.de>
Subject: Re: [PATCH] mmap_vmcore: skip non-ram pages reported by hypervisors
Vivek Goyal <vgoyal@...hat.com> writes:
> 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?
For Xen pfn_is_ram() returns 0 in case the page is an mmio. Ballooned
pages are also considered being mmio (HVMOP_get_mem_type returns
HVMMEM_mmio_dm).
>
> If yes, then page_is_zero_filled() might be a more approprate name.
>
It's not as mmio page is not always zero-filled. We just don't need
these pages in vmcore.
> Also I am wondering why it was not done as part of copy_oldmem_page()
> so that respective arch could hide all the details.
>
Afaiac that wouldn't solve the mmap issue I'm trying to address but we
can ask Olaf why he preferred pfn_is_ram() path.
>> 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?
>
I meant to say that we have many hypervisors for x86 supported. In case
I override __weak remap_oldmem_pfn_range() in xen-specific code it will
*always* get executed when this code was compiled. In case we'll have to
do similar override in e.g. Hyperv or KVM code in future we'll have a
mess (in which order do we need to execute these overrides?).
In few words, Xen-PVHVM is not an architecture so I'm not following
"Architectures may override this function to map oldmem" path.
>>
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>> ---
>> 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.
I wanted to preserve the direct path without the check to make things
faster when the pfn_is_ram() handler was not registered. oldmem is huge
sometimes and issuing a single call per pfn can cost us something.
>
> Thanks
> Vivek
--
Vitaly
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