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 19:24:43 +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

Hi Michał,

Thank you so much for reviewing.

On 11/7/22 5:26 PM, Michał Mirosław wrote:
>> +
>> +/*
>> + * struct page_region - Page region with bitmap flags
>> + * @start:     Start of the region
>> + * @len:       Length of the region
>> + * bitmap:     Bits sets for the region
>> + */
>> +struct page_region {
>> +       __u64 start;
>> +       __u64 len;
>> +       __u32 bitmap;
>> +       __u32 __reserved;
> 
> "u64 flags"? If an extension is needed it would already require a new
> ioctl or something in the `arg` struct.
I feel like the masks must have the same type as this bitmap variable as 
the return_mask specifies the flags to be returned in bitmap. All the 
masks are of type __u32. This is why I'd kept the bitmap of type _u32 as 
well. I've kept them of 32 bit size as currently we are adding support 
for 4 flags and there is still room to add 28 more bits in the future. 
Do you still think that I should update the masks and bitmap to _u64?

>> + * @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 only reviewed the API now. The implementation I think could be
> simpler, but let's leave that to after the API is agreed on.)
> 
> Best Regards
> Michał Mirosław