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

[Muhammad Usama Anjum <usama.anjum@collabora.com>]

On 2/21/23 5:42 PM, Michał Mirosław 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).
At minimum, one mask (required, any or excluded) must be specified. For a
page to get selected, the page flags must fulfill the criterion of all the
specified masks.

If a flag is present in both required_mask and excluded_mask, the
required_mask would select a page. But exculded_mask would drop the page.
So page page would be dropped. It is responsibility of the user to
correctly specify the flags.

matched = true;
if (p->required_mask)
	matched = ((p->required_mask & bitmap) == p->required_mask);
if (matched && p->anyof_mask)
	matched = (p->anyof_mask & bitmap);
if (matched && p->excluded_mask)
	matched = !(p->excluded_mask & bitmap);

if (matched && bitmap) {
	// page selected
}

Do you accept/like this behavior of masks after explaintation?

> (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.
Lets translate words into table:
pageflags	required_flags	negated_flags	matched
1		1		0		yes
0		1		1		yes

> 
> 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

pageflags	anyof_flags	negated_flags	matched
1		1		0		yes
0		1		1		yes

> 
> 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? 
All the masks are optional. If all or any of the 3 masks are specified, the
page flags must pass these masks to get selected.

> 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;
> 
After taking a while to understand this and compare with already present
flag system, `negated flags` is comparatively difficult to understand while
already present flags seem easier.

> 
> Best Regards
> Michał Mirosław

-- 
BR,
Muhammad Usama Anjum