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

[Michał Mirosław]

On Fri, 24 Feb 2023 at 03:20, Andrei Vagin <avagin@xxxxxxxxx> wrote:
>
> On Tue, Feb 21, 2023 at 4:42 AM Michał Mirosław <emmir@xxxxxxxxxx> wrote:
> >
> > On Tue, 21 Feb 2023 at 11:28, Muhammad Usama Anjum
> > <usama.anjum@xxxxxxxxxxxxx> 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.

Hi,

I would not want the discussion to wander into easier/harder territory
as that highty depends on experience one has. What I'm arguing about
is the consistency of the API. Let me expand a bit on that.

We have two ways to look at the page_flags:
 A. the field represents a *set of elements* (tags, attributes)
present on the page;
 B. the field represents a bitfield (structure; a fixed set of boolean
fields having a value of 0 or 1)

>From A follows the include/exclude way of API design for matching the
flags, and from B the matched mask (which flags to check) + value set
(what values to require).

My argument is that B is consistent with how the flags are used in the
kernel: we don't have operations that add or remove flags, but we have
operations that set or change their value.

Best Regards
Michał Mirosław