[CRIU] [PATCH 5/9] Handle vmsplice failure for read mode pre-dump

[abhishek dubey]

Yes, this is something for which I had no clue. So I fixed it with minor 
hack by writing 1 page less to pipe.

Fixing it will require deep debugging.

On 25/09/19 9:57 PM, Andrei Vagin wrote:
> On Sun, Sep 22, 2019 at 09:54:54AM +0530, Abhishek Dubey wrote:
>> vmsplice fails to completely splice 2^k pages
>> from user-buffer to ppb pipes. Sometimes last
>> page is partially spliced.
>> To overcome this, page count for splicing is
>> kept 1 less than the size of pipe.
>>
>> Signed-off-by: Abhishek Dubey <dubeyabhishek777 at gmail.com>
>> ---
>>   criu/page-pipe.c | 16 ++++++++++++++++
>>   1 file changed, 16 insertions(+)
>>
>> diff --git a/criu/page-pipe.c b/criu/page-pipe.c
>> index a821696..e60dd85 100644
>> --- a/criu/page-pipe.c
>> +++ b/criu/page-pipe.c
>> @@ -40,6 +40,22 @@ static int __ppb_resize_pipe(struct page_pipe_buf *ppb, unsigned long new_size)
>>   	ret /= PAGE_SIZE;
>>   	BUG_ON(ret < ppb->pipe_size);
>>   
>> +	/*
>> +	 * TODO: Investigate and handle
>> +	 * Sometimes vmsplice fails to splice N pages(where N=2^k)
>> +	 * from user buffer to pipe, so restricting page-pipe
>> +	 * capacity to accommodate N-1 pages.
>> +	 * e.g. Pipe size can be 512 pages, but it's filled till
>> +	 * 511 pages.
>> +	 *
>> +	 * Steps to reproduce issue:
>> +	 * 1. Comment-out next two lines following this comment
>> +	 * 2. Run maps007/maps004 from transitive tests with
>> +	 *    --pre-dump-mode=read option
>> +	 */
>> +	if (opts.pre_dump_mode == PRE_DUMP_READ)
>> +		ret -= 1;
> This doesn't look nice...
>
>> +
>>   	pr_debug("Grow pipe %x -> %x\n", ppb->pipe_size, ret);
>>   	ppb->pipe_size = ret;
>>   
>> -- 
>> 2.7.4
>>