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

[Michał Mirosław]

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