On 2/21/23 5:42 PM, Michał Mirosław 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). 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