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
