Re: [PATCH v10 3/6] fs/proc/task_mmu: Implement IOCTL to get and/or the clear info about PTEs

[Andrei Vagin <avagin@gmail.com>]

On Tue, Feb 21, 2023 at 4:42 AM Michał Mirosław <emmir@google.com> wrote:
>
> On Tue, 21 Feb 2023 at 11:28, Muhammad Usama Anjum
> <usama.anjum@collabora.com> wrote:
> >
> > Hi Michał,
> >
> > Thank you so much for comment!
> >
> > On 2/17/23 8:18 PM, Michał Mirosław wrote:
> [...]
> > > For the page-selection mechanism, currently required_mask and
> > > excluded_mask have conflicting
> > They are opposite of each other:
> > All the set bits in required_mask must be set for the page to be selected.
> > All the set bits in excluded_mask must _not_ be set for the page to be
> > selected.
> >
> > > responsibilities. I suggest to rework that to:
> > > 1. negated_flags: page flags which are to be negated before applying
> > > the page selection using following masks;
> > Sorry I'm unable to understand the negation (which is XOR?). Lets look at
> > the truth table:
> > Page Flag       negated_flags
> > 0               0                       0
> > 0               1                       1
> > 1               0                       1
> > 1               1                       0
> >
> > If a page flag is 0 and negated_flag is 1, the result would be 1 which has
> > changed the page flag. It isn't making sense to me. Why the page flag bit
> > is being fliped?
> >
> > When Anrdei had proposed these masks, they seemed like a fancy way of
> > filtering inside kernel and it was straight forward to understand. These
> > masks would help his use cases for CRIU. So I'd included it. Please can you
> > elaborate what is the purpose of negation?
>
> The XOR is a way to invert the tested value of a flag (from positive
> to negative and the other way) without having the API with invalid
> values (with required_flags and excluded_flags you need to define a
> rule about what happens if a flag is present in both of the masks -
> either prioritise one mask over the other or reject the call).
> (Note: the XOR is applied only to the value of the flags for the
> purpose of testing page-selection criteria.)

Michał,

Your API isn't much different from the current one, but it requires
a bit more brain activity for understanding.

The current set of masks can be easy translated to the new one:
negated_flags = excluded_flags
required_flags_new = excluded_flags | required_flags

As for invalid values, I think it is an advantage of the current API.
I mean we can easily detect invalid values and return EINVAL. With your
API, such mistakes will be undetectable.

As for priorities, I don't see this problem here If I don't miss something.

We can rewrite the code this way:
```
if (required_mask && ((page_flags & required_mask) != required_mask)
  skip page;
if (anyof_mask && !(page_flags & anyof_mask))
  skip page;
if (page_flags & excluded_mask)
  skip page;
```

I think the result is always the same no matter in what order each
mask is applied.

Thanks,
Andrei