[CRIU] [PATCH v4 07/12] criu: page-xfer: add ability to skip writing lazy pages

[Pavel Emelyanov]

On 06/09/2016 05:18 PM, Mike Rapoport wrote:
> On Thu, Jun 9, 2016 at 4:49 PM, Pavel Emelyanov <xemul at virtuozzo.com> wrote:
>> On 06/09/2016 04:42 PM, Mike Rapoport wrote:
>>> On Thu, Jun 9, 2016 at 4:39 PM, Pavel Emelyanov <xemul at virtuozzo.com> wrote:
>>>> On 06/09/2016 04:24 PM, Mike Rapoport wrote:
>>>>> On Thu, Jun 9, 2016 at 4:22 PM, Pavel Emelyanov <xemul at virtuozzo.com> wrote:
>>>>>> On 06/09/2016 04:12 PM, Mike Rapoport wrote:
>>>>>>> On Thu, Jun 9, 2016 at 4:12 PM, Mike Rapoport <mike.rapoport at gmail.com> wrote:
>>>>>>>>>
>>>>>>>>> I have a question here -- if page_xfer_dump_pages sends a pagemap the receiver
>>>>>>>>> would expect the subsequent pages on the socket, but instead, it _may_ receive
>>>>>>>>> next pagemap if the previous one was lazy. How it this handled?
>>>>>>>>
>>>>>>>> Hmmm. It is not... I think that page-server and, most probably,
>>>>>>>> incremental dumps are not compatible with lazy-pages...
>>>>>>>
>>>>>>> As of now :(
>>>>>>
>>>>>> OK... And when in the lazy-pages code-flow would this page_xfer_dump_pages()
>>>>>> get called?
>>>>>
>>>>> At the "usual place", dump_pages will have yet another boolean
>>>>> parameter 'lazy' that will be passed to page_xfer_dump_pages()
>>>>
>>>> OK, so we do "regular" dump that doesn't write lazy pages into pages.img, right?
>>>
>>> Yes
>>
>> OK, this creates ... broken pagemap/pages image set, as pagemap is present,
>> but the respective page-set is not :( Can we add optional bool lazy bit
>> for this?
> 
> I'd add entire 'u32 flags' for upward compatibility :)

In protobuf bools work just fine :)

>> AFAIU the restore side reads the pagemap and understands that
>> it's lazy by looking at VMA, does it?
> 
> Yep, the restore side reads pagemap, checks if the vma can be lazy and
> skips the pages. The page read has a hack to avoid lseek of pages.img.
> The whole think is kinda hackish...

Indeed :) Let's try to add bool lazy on pagemap entry, things should get simpler.

-- Pavel