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

["Michał Mirosław" <emmir@google.com>]

On Thu, 23 Feb 2023 at 07:44, Muhammad Usama Anjum
<usama.anjum@collabora.com> wrote:
>
> On 2/22/23 4:48 PM, Michał Mirosław wrote:
> > On Wed, 22 Feb 2023 at 12:06, Muhammad Usama Anjum
> > <usama.anjum@collabora.com> wrote:
[...]
> >>>>> 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.

I'm not sure why do we want uniformity in the case of 1 flag? If a
user specifies a single required flag, I'd expect he doesn't need to
look at the flags returned as those will duplicate the information
from mere presence of a page. A user might also require a single flag,
but want all of them returned. Both requests - return 1 flag and
return 0 flags would give meaningful output, so why force one way or
the other? Allowing two will also enable users to express the intent:
they need either just a list of pages, or they need a list with
per-page flags - the need would follow from the code structure or
other factors.

> >>>> 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.

I require flags PAGE_IS_WRITTEN=1, PAGE_IS_FILE=0, so:

required_mask = PAGE_IS_WRITTEN | PAGE_IS_FILE;
negated_flags = PAGE_IS_FILE; // flags I want zero

I also require one of PAGE_IS_PRESENT=1 or PAGE_IS_SWAP=1, so:

anyof_mask = PAGE_IS_PRESENT | PAGE_IS_SWAP;

Another case: I want to analyse a process' working set:

required_mask = 0;
negated_flags = PAGE_IS_FILE;
anyof_mask = PAGE_IS_FILE | PAGE_IS_WRITTEN;

-> gathering pages modified [WRITTEN=1] or not backed by a file [FILE=0].

To clarify a bit: negated_flags doesn't mask anything: the field
inverts values of the flags (marks some "active low", if you consider
electronic signal analogy).

Best Regards
Michał Mirosław