Re: [PATCH v18 2/5] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs

[Michał Mirosław]

On Thu, 15 Jun 2023 at 17:11, Muhammad Usama Anjum
<usama.anjum@xxxxxxxxxxxxx> wrote:
> On 6/15/23 7:52 PM, Michał Mirosław wrote:
> > On Thu, 15 Jun 2023 at 15:58, Muhammad Usama Anjum
> > <usama.anjum@xxxxxxxxxxxxx> wrote:
> >> I'll send next revision now.
> >> On 6/14/23 11:00 PM, Michał Mirosław wrote:
> >>> (A quick reply to answer open questions in case they help the next version.)
> >>>
> >>> On Wed, 14 Jun 2023 at 19:10, Muhammad Usama Anjum
> >>> <usama.anjum@xxxxxxxxxxxxx> wrote:
> >>>> On 6/14/23 8:14 PM, Michał Mirosław wrote:
> >>>>> On Wed, 14 Jun 2023 at 15:46, Muhammad Usama Anjum
> >>>>> <usama.anjum@xxxxxxxxxxxxx> wrote:
> >>>>>>
> >>>>>> On 6/14/23 3:36 AM, Michał Mirosław wrote:
> >>>>>>> On Tue, 13 Jun 2023 at 12:29, Muhammad Usama Anjum
> >>>>>>> <usama.anjum@xxxxxxxxxxxxx> wrote:
> >>> [...]
> >>>>>>>> +       if (cur_buf->bitmap == bitmap &&
> >>>>>>>> +           cur_buf->start + cur_buf->len * PAGE_SIZE == addr) {
> >>>>>>>> +               cur_buf->len += n_pages;
> >>>>>>>> +               p->found_pages += n_pages;
> >>>>>>>> +       } else {
> >>>>>>>> +               if (cur_buf->len && p->vec_buf_index >= p->vec_buf_len)
> >>>>>>>> +                       return -ENOMEM;
> >>>>>>>
> >>>>>>> Shouldn't this be -ENOSPC? -ENOMEM usually signifies that the kernel
> >>>>>>> ran out of memory when allocating, not that there is no space in a
> >>>>>>> user-provided buffer.
> >>>>>> There are 3 kinds of return values here:
> >>>>>> * PM_SCAN_FOUND_MAX_PAGES (1) ---> max_pages have been found. Abort the
> >>>>>> page walk from next entry
> >>>>>> * 0 ---> continue the page walk
> >>>>>> * -ENOMEM --> Abort the page walk from current entry, user buffer is full
> >>>>>> which is not error, but only a stop signal. This -ENOMEM is just
> >>>>>> differentiater from (1). This -ENOMEM is for internal use and isn't
> >>>>>> returned to user.
> >>>>>
> >>>>> But why ENOSPC is not good here? I was used before, I think.
> >>>> -ENOSPC is being returned in form of true error from
> >>>> pagemap_scan_hugetlb_entry(). So I'd to remove -ENOSPC from here as it
> >>>> wasn't true error here, it was only a way to abort the walk immediately.
> >>>> I'm liking the following erturn code from here now:
> >>>>
> >>>> #define PM_SCAN_BUFFER_FULL     (-256)
> >>>
> >>> I guess this will be reworked anyway, but I'd prefer this didn't need
> >>> custom errors etc. If we agree to decoupling the selection and GET
> >>> output, it could be:
> >>>
> >>> bool is_interesting_page(p, flags); // this one does the
> >>> required/anyof/excluded match
> >>> size_t output_range(p, start, len, flags); // this one fills the
> >>> output vector and returns how many pages were fit
> >>>
> >>> In this setup, `is_interesting_page() && (n_out = output_range()) <
> >>> n_pages` means this is the final range, no more will fit. And if
> >>> `n_out == 0` then no pages fit and no WP is needed (no other special
> >>> cases).
> >> Right now, pagemap_scan_output() performs the work of both of these two
> >> functions. The part can be broken into is_interesting_pages() and we can
> >> leave the remaining part as it is.
> >>
> >> Saying that n_out < n_pages tells us the buffer is full covers one case.
> >> But there is case of maximum pages have been found and walk needs to be
> >> aborted.
> >
> > This case is exactly what `n_out < n_pages` will cover (if scan_output
> > uses max_pages properly to limit n_out).
> > Isn't it that when the buffer is full we want to abort the scan always
> > (with WP if `n_out > 0`)?
> Wouldn't it be duplication of condition if buffer is full inside
> pagemap_scan_output() and just outside it. Inside pagemap_scan_output() we
> check if we have space before putting data inside it. I'm using this same
> condition to indicate that buffer is full.

I'm not sure what do you mean? The buffer-full conditions would be
checked in ..scan_output() and communicated to the caller by returning
N less than `n_pages` passed in. This is exactly how e.g. read()
works: if you get less than requested you've hit the end of the file.
If the file happens to have size that is equal to the provided buffer
length, the next read() will return 0.

> >>>>> While here, I wonder if we really need to fail the call if there are
> >>>>> unknown bits in those masks set: if this bit set is expanded with
> >>>>> another category flags, a newer userspace run on older kernel would
> >>>>> get EINVAL even if the "treat unknown as 0" be what it requires.
> >>>>> There is no simple way in the API to discover what bits the kernel
> >>>>> supports. We could allow a no-op (no WP nor GET) call to help with
> >>>>> that and then rejecting unknown bits would make sense.
> >>>> I've not seen any examples of this. But I've seen examples of returning
> >>>> error if kernel doesn't support a feature. Each new feature comes with a
> >>>> kernel version, greater than this version support this feature. If user is
> >>>> trying to use advanced feature which isn't present in a kernel, we should
> >>>> return error and not proceed to confuse the user/kernel. In fact if we look
> >>>> at userfaultfd_api(), we return error immediately if feature has some bit
> >>>> set which kernel doesn't support.
> >>>
> >>> I think we should have a way of detecting the supported flags if we
> >>> don't want a forward compatibility policy for flags here. Maybe it
> >>> would be enough to allow all the no-op combinations for this purpose?
> >> Again I don't think UFFD is doing anything like this.
> >
> > If it's cheap and easy to provide a user with a way to detect the
> > supported features - why not do it?
> I'm sorry. But it would bring up something new and iterations will be
> needed to just play around. I like the UFFD way.

Let's then first agree on what would have to be changed. I guess we
could leverage that `scan_len = 0` doesn't make much sense otherwise
and let it be used to check the other fields for support.

Best Regards
Michał Mirosław