Re: [PATCH v11 0/7] Implement IOCTL to get and optionally clear info about PTEs

Andrei Vagin
2023-03-20T18:38:46Z

On Thu, Mar 9, 2023 at 11:58 AM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Thu, 9 Mar 2023 18:57:11 +0500 Muhammad Usama Anjum <usama.anjum@collabora.com> wrote:
>
> > The information related to pages if the page is file mapped, present and
> > swapped is required for the CRIU project [5][6]. The addition of the
> > required mask, any mask, excluded mask and return masks are also required
> > for the CRIU project [5].
>
> It's a ton of new code and what I'm not seeing in here (might have
> missed it?) is a clear statement of the value of this feature to our
> users.
>
> I see hints that CRIU would like it, but no description of how valuable
> this is to CRIU's users.

Hi Andrew,

The current interface works for CRIU, and I can't say we have anything
critical with it right now.

On the other hand, the new interface has a number of significant improvements:

* it is more granular and allows us to track changed pages more
effectively. The current interface can clear dirty bits for the entire
process only. In addition, reading info about pages is a separate
operation. It means we must freeze the process to read information
about all its pages, reset dirty bits, only then we can start dumping
pages. The information about pages becomes more and more outdated,
while we are processing pages. The new interface solves both these
downsides. First, it allows us to read pte bits and clear the
soft-dirty bit atomically. It means that CRIU will not need to freeze
processes to pre-dump their memory. Second, it clears soft-dirty bits
for a specified region of memory. It means CRIU will have actual info
about pages to the moment of dumping them.

* The new interface has to be much faster because basic page filtering
is happening in the kernel. With the old interface, we have to read
pagemap for each page.

Thanks,
Andrei

>
> So please spend some time preparing this info.
>
> Also, are any other applications of this feature anticipated? If so,
> what are they?
>
> IOW, please sell this stuff to us!