[CRIU] [PATCH] Add test case for code page sharing

[Christopher Covington]

On 04/07/2015 09:27 AM, Pavel Emelyanov wrote:
> 
>>>> and restore only changed pages.
>>>>
>>>> static inline bool should_dump_page(VmaEntry *vmae, u64 pme)
>>>> {
>>>> ...
>>>>         /*
>>>>          * Optimisation for private mapping pages, that haven't
>>>>          * yet being COW-ed
>>>>          */
>>>>         if (vma_entry_is(vmae, VMA_FILE_PRIVATE) && (pme & PME_FILE))
>>>>                 return false;
>>>
>>> But that's the dump routine. For COW restore different code is used:
>>
>> No. It's another case. We are talking about private file mappings. If
>> processes don't change content, they access shared pages which are
>> associated with a file. If a process tries to change a page, the kernel
>> allocates a new page and copies the file page into the new page.
> 
> Ah, I see. Well, yes, CRIU touches one private executable page to inject
> parasite routine into it. Then this page gets dumped into the image.
> 
> Are we talking about a single page here?

A single page losing sharing because it was ptrace poked is a small concern.
The sharing being maintained for that single page would be "nice to have" but
I don't see it as a high priority.

The greater concern is that all of the ELF's read-only pages, even those never
poked by ptrace, may be losing sharing. We're going to extend the test case to
span several pages to confirm or disprove our reading of the code. I'm not
sure Andrew's extension of the test to look at getpagesize() disproves the
concern, because that library function is in a separate .so file rather than
part of the main ELF file.

Chris

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