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

["Michał Mirosław" <emmir@google.com>]

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.)

So:
1. if a flag is not set in negated_flags, but set in required_flags,
then it means "this flag must be one" - equivalent to it being set in
required_flag (in your current version of the API).
2. if a flag is set in negated_flags and also in required_flags, then
it means "this flag must be zero" - equivalent to it being set in
excluded_flags.

The same thing goes for anyof_flags: if a flag is set in anyof_flags,
then for it to be considered matched:
1. it must have a value of 1 if it is not set in negated_flags
2. it must have a value of 0 if it is set in negated_flags

BTW, I think I assumed that both conditions (all flags in
required_flags and at least one in anyof_flags is present) need to be
true for the page to be selected - is this your intention? The example
code has a bug though, in that if anyof_flags is zero it will never
match. Let me fix the selection part:

// calc. a mask of flags that have expected ("active") values
tested_flags = page_flags ^ negated_flags;
// are all required flags in "active" state? [== all zero when negated]
if (~tested_flags & required_mask)
  skip page;
// is any extra flag "active"?
if (anyof_flags && !(tested_flags & anyof_flags))
  skip page;


Best Regards
Michał Mirosław