[CRIU] [PATCH v2 3/3] aio: Restore aio ring content

[Pavel Emelyanov]

On 03/18/2016 01:31 PM, Kirill Tkhai wrote:
> 
> 
> On 17.03.2016 22:34, Pavel Emelyanov wrote:
>>
>>>>>> I'm not sure this is safe. How would pre-dumps act on rings?
>>>>>
>>>>> Could you please explain what kind of problems are possible here?
>>>>> I don't see a memory predump.
>>>>
>>>> The vma_entry_is_private() check is too generic. E.g. such vmas are being
>>>> soft-dirty-tracked. Do we want the same for AIO rings? I bet we don't :)
>>>
>>> For user AIO ring buffer looks like an anonymous memory. There are no difference
>>> between them, it's writable and modifiable. So if we track anonymous memory,
>>> we have to track AIO ring buffer too.
>>
>> Will it get tracked by the kernel's soft-dirty bits? I heavily doubt it.
> 
> It's tracked. Below is the prove.
> 
> #define _GNU_SOURCE
> #include <stdio.h>
> #include <unistd.h>
> #include <sys/syscall.h>
> #include <linux/aio_abi.h>
> #include <fcntl.h>
> #include <inttypes.h>
> 
> inline int io_setup(unsigned nr, aio_context_t *ctxp)
> {
> 	return syscall(__NR_io_setup, nr, ctxp);
> }
> 
> #define PME_SOFT_DIRTY	(1ULL << 55)
> #define PAGE_SHIFT     12
> #define PAGE_SIZE      (1UL << PAGE_SHIFT)
> #define u64 uint64_t
> 
> int main()
> {
> 	aio_context_t ctx = 0;
> 	int ret, fd, pm2;
> 	u64 pmap;
> 
> 	ret = io_setup(128, &ctx);
> 	if (ret < 0) {
> 		perror("io_setup error");
> 		return -1;
> 	}
> 
> 	fd = open("/proc/self/clear_refs", O_WRONLY);
> 	if (fd < 0) {
> 		perror("clear_refs open");
> 		return -1;
> 	}
> 
> 	if (write(fd, "4", 1) != 1) {
> 		perror("clear_refs write");
> 		return -1;
> 	}
> 	close(fd);
> 
> 	pm2 = open("/proc/self/pagemap", O_RDONLY);
> 	if (pm2 < 0) {
> 		perror("Can't open pagemap file");
> 		return -1;
> 	}
> 
> 	((char *)ctx)[0] = '\0';
> 	lseek(pm2, ctx / PAGE_SIZE * sizeof(u64), SEEK_SET);
> 	ret = read(pm2, &pmap, sizeof(pmap));
> 	if (ret < 0)
> 		perror("Read pmap err!");
> 	close(pm2);
> 	if (pmap & PME_SOFT_DIRTY)
> 		printf("Dirty tracking exists on aio\n");
> 	else
> 		printf("Shit happens\n");

That's not prove. Kernel also updates the ring when completing requests, but
you don't check this case.

Anyway, I don't think treating aio ring buffer as regular anonymous memory
is good idea.

> 
> 	return 0;
> }