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

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Date:   Tue, 8 Nov 2022 22:51:04 +0500
From:   Muhammad Usama Anjum <usama.anjum@...labora.com>
To:     Michał Mirosław <emmir@...gle.com>
Cc:     Muhammad Usama Anjum <usama.anjum@...labora.com>,
        Andrei Vagin <avagin@...il.com>,
        Danylo Mocherniuk <mdanylo@...gle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Christian Brauner <brauner@...nel.org>,
        Peter Xu <peterx@...hat.com>, Yang Shi <shy828301@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Zach O'Keefe <zokeefe@...gle.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>, kernel@...labora.com,
        Gabriel Krisman Bertazi <krisman@...labora.com>,
        David Hildenbrand <david@...hat.com>,
        Peter Enderborg <peter.enderborg@...y.com>,
        "open list : KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>, Shuah Khan <shuah@...nel.org>,
        open list <linux-kernel@...r.kernel.org>,
        "open list : PROC FILESYSTEM" <linux-fsdevel@...r.kernel.org>,
        "open list : MEMORY MANAGEMENT" <linux-mm@...ck.org>
Subject: Re: [PATCH v5 2/3] fs/proc/task_mmu: Implement IOCTL to get and/or
 the clear info about PTEs

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