On Wed, 22 Feb 2023 at 12:06, Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx> wrote: > > On 2/22/23 3:44 PM, Michał Mirosław wrote: > > On Wed, 22 Feb 2023 at 11:11, Muhammad Usama Anjum > > <usama.anjum@xxxxxxxxxxxxx> wrote: > >> 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. > > > > [Please see the comment below.] > > > > [...] > >> Lets translate words into table: > > [Yes, those tables captured the intent correctly.] > > > >>> 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. > > > > This explanation contradicts in part the introductory paragraph, but > > this version seems more useful as you can pass all masks zero to have > > all pages selected. > Sorry, I wrote it wrongly. (All the masks are not optional.) Let me > rephrase. All or at least any 1 of the 3 masks (required, any, exclude) > must be specified. The return_mask must always be specified. Error is > returned if all 3 masks (required, anyof, exclude) are zero or return_mask > is zero. Why do you need those restrictions? I'd guess it is valid to request a list of all pages with zero return_mask - this will return a compact list of used ranges of the virtual address space. > >> 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. > > > > Maybe replacing negated_flags in the API with matched_values = > > ~negated_flags would make this better? > > > > We compare having to understand XOR vs having to understand ordering > > of required_flags and excluded_flags. > There is no ordering in current masks scheme. No mask is preferable. For a > page to get selected, all the definitions of the masks must be fulfilled. > You have come up with good example that what if required_mask = > exclude_mask. In this case, no page will fulfill the criterion and hence no > page would be selected. It is user's fault that he isn't understanding the > definitions of these masks correctly. > > Now thinking about it, I can add a error check which would return error if > a bit in required and excluded masks matches. Would you like it? Lets put > this check in place. > (Previously I'd left it for user's wisdom not to do this. If he'll specify > same masks in them, he'll get no addresses out of the syscall.) This error case is (one of) the problems I propose avoiding. You also need much more text to describe the requred/excluded flags interactions and edge cases than saying that a flag must have a value equal to corresponding bit in ~negated_flags to be matched by requried/anyof masks. > > IOW my proposal is to replace branches in the masks interpretation (if > > in one set then matches but if in another set then doesn't; if flags > > match ... ) with plain calculation (flag is matching when equals > > ~negated_flags; if flags match the masks ...). Best Regards Michał Mirosław
