On 2/22/23 4:48 PM, Michał Mirosław wrote: > 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. At the time, we are supporting 4 flags (PAGE_IS_WRITTEN, PAGE_IS_FILE, PAGE_IS_PRESENT and PAGE_IS_SWAPPED). The idea is that user mention his flags of interest in the return_mask. If he wants only 1 flag, he'll specify it. Definitely if user wants only 1 flag, initially it doesn't make any sense to mention in the return mask. But we want uniformity. If user want, 2 or more flags in returned, return_mask becomes compulsory. So to keep things simple and generic for any number of flags of interest returned, the return_mask must be specified even if the flag of interest is only 1. > >>>> 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. I've found excluded_mask very intuitive as compared to negated_mask which is so difficult to understand that I don't know how to use it correctly. Lets take an example, I want pages which are PAGE_IS_WRITTEN and are not PAGE_IS_FILE. In addition, the pages must be PAGE_IS_PRESENT or PAGE_IS_SWAPPED. This can be specified as: required_mask = PAGE_IS_WRITTEN excluded_mask = PAGE_IS_FILE anyof_mask = PAGE_IS_PRESETNT | PAGE_IS_SWAP (a) assume page_flags = 0b1111 skip page as 0b1111 & 0b0010 = true (b) assume page_flags = 0b1001 select page as 0b1001 & 0b0010 = false It seemed intuitive. Right? How would you achieve same thing with negated_mask? required_mask = PAGE_IS_WRITTEN negated_mask = PAGE_IS_FILE anyof_mask = PAGE_IS_PRESETNT | PAGE_IS_SWAP (1) assume page_flags = 0b1111 tested_flags = 0b1111 ^ 0b0010 = 0b1101 (2) assume page_flags = 0b1001 tested_flags = 0b1001 ^ 0b0010 = 0b1011 In (1), we wanted to skip pages which have PAGE_IS_FILE set. But negated_mask has just masked it and page is still getting tested if it should be selected and it would get selected. It is wrong. In (2), the PAGE_IS_FILE bit of page_flags was 0 and got updated to 1 or PAGE_IS_FILE in tested_flags. > >>> 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 -- BR, Muhammad Usama Anjum
