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

[Muhammad Usama Anjum]

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.

> 
>>>>>>> For flags name: PM_REQUIRE_WRITE_ACCESS?
>>>>>>> Or Is it intended to be checked only if doing WP (as the current name
>>>>>>> suggests) and so it would be redundant as WP currently requires
>>>>>>> `p->required_mask = PAGE_IS_WRITTEN`?
>>>>>> This is intended to indicate that if userfaultfd is needed. If
>>>>>> PAGE_IS_WRITTEN is mentioned in any of mask, we need to check if
>>>>>> userfaultfd has been initialized for this memory. I'll rename to
>>>>>> PM_SCAN_REQUIRE_UFFD.
>>>>>
>>>>> Why do we need that check? Wouldn't `is_written = false` work for vmas
>>>>> not registered via uffd?
>>>> UFFD_FEATURE_WP_ASYNC and UNPOPULATED needs to be set on the memory region
>>>> for it to report correct written values on the memory region. Without UFFD
>>>> WP ASYNC and UNPOUPULATED defined on the memory, we consider UFFD_WP state
>>>> undefined. If user hasn't initialized memory with UFFD, he has no right to
>>>> set is_written = false.
>>>
>>> How about calculating `is_written = is_uffd_registered() &&
>>> is_uffd_wp()`? This would enable a user to apply GET+WP for the whole
>>> address space of a process regardless of whether all of it is
>>> registered.
>> I wouldn't want to check if uffd is registered again and again. This is why
>> we are doing it only once every walk in pagemap_scan_test_walk().
> 
> There is no need to do the checks repeatedly. If I understand the code
> correctly, uffd registration is per-vma, so it can be communicated
> from test_walk to entry/hole callbacks via a field in
> pagemap_scan_private.
> 
>>>>> 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.
> 
> Best Regards
> Michał Mirosław

-- 
BR,
Muhammad Usama Anjum