[CRIU] [RFC PATCH] Try to include userfaultfd with criu

[Adrian Reber]

On Mon, Oct 26, 2015 at 10:58:49PM +0300, Pavel Emelyanov wrote:
> On 10/26/2015 10:49 PM, Adrian Reber wrote:
> > On Mon, Oct 26, 2015 at 07:13:06PM +0300, Pavel Emelyanov wrote:
> >> On 10/26/2015 03:41 PM, Adrian Reber wrote:
> >>> Pavel, thanks for the comments on the userfaultfd integration. I am
> >>> trying to integrate all your suggestions. I have question about
> >>> restoring pages... Or rather about not restoring pages.
> >>>
> >>> My current approach is, if the parameter --lazy-pages is specified, to
> >>> skip pages with MAP_ANONYMOUS. I thought about skipping the pages in
> >>> restore_priv_vma_content(). 
> >>
> >> Correct.
> >>
> >>> If I skip pages I need to seek with a
> >>> different value then '0' in read_pagemap_page() to read from the right
> >>> position from the pages-x.img file.
> >>
> >> Yes, there's skip_pagemap_pages() call for that :)
> > 
> > So what do you say to a patch like this:
> > 
> > diff --git a/include/page-read.h b/include/page-read.h
> > index 9e3013a..e973426 100644
> > --- a/include/page-read.h
> > +++ b/include/page-read.h
> > @@ -51,6 +51,7 @@ struct page_read {
> >         /* stop working on current pagemap */
> >         void (*put_pagemap)(struct page_read *);
> >         void (*close)(struct page_read *);
> > +       void (*skip_pages)(struct page_read *, unsigned long len);
> >  
> >         /* Private data of reader */
> >         struct cr_img *pmi;
> > diff --git a/page-read.c b/page-read.c
> > index 832c057..6c0bb95 100644
> > --- a/page-read.c
> > +++ b/page-read.c
> > @@ -279,6 +279,7 @@ int open_page_read_at(int dfd, int pid, struct page_read *pr, int pr_flags)
> >         pr->put_pagemap = put_pagemap;
> >         pr->read_page = read_pagemap_page;
> >         pr->close = close_page_read;
> > +       pr->skip_pages = skip_pagemap_pages;
> >         pr->id = ids++;
> >  
> >         pr_debug("Opened page read %u (parent %u)\n",
> > @@ -301,6 +302,7 @@ open_old:
> >         pr->read_page = read_page;
> >         pr->pi = NULL;
> >         pr->close = close_page_read;
> > +       pr->skip_pages = NULL;
> >  
> >         return 1;
> >  }
> > 
> > This way I can use skip pages in cr-restore.c and I am only reading
> > the pages I cannot restore via uffd. I could submit this as proper
> > patch independent of the uffd functionality.
> 
> I'm not 100% sure that just calling this one will work for your case. The
> skip_pagemap_pages() skips only in current pagemap and if you _can_ 
> overlap in your code, then this won't work. The seek_pagemap_pages() is
> higher level routine that can jump to the next pagemap in the sequence
> (again -- if your code needs it).
> 
> And even worse %) -- if you have stacked pagemap images you should
> correctly skip data from lower levels too, the read_pagemap_page() does
> _this_, so if your code doesn't want to mess with it, you should write
> something similar.
> 
> In the end -- the exact page_read method you'd need to add would depend
> on how exactly you access the images and skip data in it.

Okay, good to know. Thanks. I will have to find out what really need.
For my simple test case it works, but good to know that it will probably 
break sooner than later.

		Adrian