Re: [PATCH v5 2/3] fs/proc/task_mmu: Implement IOCTL to get and/or the clear info about PTEs

[Muhammad Usama Anjum <usama.anjum@collabora.com>]

On 11/8/22 9:00 PM, Michał Mirosław wrote:
>>>> + * @start:             Starting address of the region
>>>> + * @len:               Length of the region (All the pages in this length are included)
>>>> + * @vec:               Address of page_region struct array for output
>>>> + * @vec_len:           Length of the page_region struct array
>>>> + * @max_pages:         Optional max return pages (It must be less than vec_len if specified)
>>> I think we discussed that this is not counting the same things as
>>> vec_len, so there should not be a reference between the two. The limit
>>> is whatever fits under both conditions (IOW: n_vecs <= vec_len &&
>>> (!max_pages || n_pages <= max_pages).
>> In worse case when pages cannot be folded into the page_region, the one
>> page_region may have information of only one page. This is why I've
>> compared them. I want to communicate to the user that if max_pages is
>> used, the vec_len should be of equal or greater size (to cater worse
>> case which can happen at any time). Otherwise in worse case, the api can
>> return without finding the max_pages number of pages. I don't know how
>> should I put this in the comment.
> I'm not sure you need to, as this conclusion follows from the range vs
> page distinction.
> A user who wants to cater for the worst case will provide big-enough
> `vec` array, but another, who might be memory-constrained, could
> instead just retry the call with `start` updated to just after the
> last returned page until the ioctl() returns less ranges than
> `vec_len` allows.
Makes sense. I'll update and send next revision.

Thanks,
Usama
> 
> Best Regards
> Michał Mirosław