linux-kernel - Re: [PATCH v10 3/6] fs/proc/task_mmu: Implement IOCTL to get and/or the clear info about PTEs

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Message-ID: <36ddfd75-5c58-197b-16c9-9f819099ea6d@collabora.com>
Date:   Tue, 21 Feb 2023 15:28:46 +0500
From:   Muhammad Usama Anjum <usama.anjum@...labora.com>
To:     Michał Mirosław <emmir@...gle.com>,
        Andrei Vagin <avagin@...il.com>,
        Mike Rapoport <rppt@...nel.org>, Nadav Amit <namit@...are.com>
Cc:     Muhammad Usama Anjum <usama.anjum@...labora.com>,
        David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Paul Gofman <pgofman@...eweavers.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Shuah Khan <shuah@...nel.org>,
        Christian Brauner <brauner@...nel.org>,
        Yang Shi <shy828301@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Yun Zhou <yun.zhou@...driver.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Alex Sierra <alex.sierra@....com>,
        Peter Xu <peterx@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Nadav Amit <namit@...are.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
        Greg KH <gregkh@...uxfoundation.org>, kernel@...labora.com,
        Danylo Mocherniuk <mdanylo@...gle.com>
Subject: Re: [PATCH v10 3/6] fs/proc/task_mmu: Implement IOCTL to get and/or
 the clear info about PTEs

Hi Michał,

Thank you so much for comment!

On 2/17/23 8:18 PM, Michał Mirosław wrote:
> On Thu, 2 Feb 2023 at 12:30, Muhammad Usama Anjum
> <usama.anjum@...labora.com> wrote:
> [...]
>> - The masks are specified in required_mask, anyof_mask, excluded_ mask
>>   and return_mask.
> [...]

The interface was suggested by Andrei back on the review of v3 [1]:
> I mean we should be able to specify for what pages we need to get info
> for. An ioctl argument can have these four fields:
> * required bits (rmask & mask == mask) - all bits from this mask have to
be set.
> * any of these bits (amask & mask != 0) - any of these bits is set.
> * exclude masks (emask & mask == 0) = none of these bits are set.
> * return mask - bits that have to be reported to user.

> 
> May I suggest a slightly modified interface for the flags?
I've added everyone who may be interested in making interface better.

> 
> As I understand, the return_mask is what is applied to page flags to
> aggregate the list.
> This is a separate thing, and I think it doesn't need changes except
> maybe an improvement
> in the documentation and visual distinction.
> 
> For the page-selection mechanism, currently required_mask and
> excluded_mask have conflicting
They are opposite of each other:
All the set bits in required_mask must be set for the page to be selected.
All the set bits in excluded_mask must _not_ be set for the page to be
selected.

> responsibilities. I suggest to rework that to:
> 1. negated_flags: page flags which are to be negated before applying
> the page selection using following masks;
Sorry I'm unable to understand the negation (which is XOR?). Lets look at
the truth table:
Page Flag	negated_flags		
0		0			0
0		1			1
1		0			1
1		1			0

If a page flag is 0 and negated_flag is 1, the result would be 1 which has
changed the page flag. It isn't making sense to me. Why the page flag bit
is being fliped?

When Anrdei had proposed these masks, they seemed like a fancy way of
filtering inside kernel and it was straight forward to understand. These
masks would help his use cases for CRIU. So I'd included it. Please can you
elaborate what is the purpose of negation?

> 2. required_flags: flags which all have to be set in the
> (negation-applied) page flags;
> 3. anyof_flags: flags of which at least one has to be set in the
> (negation-applied) page flags;
> 
> IOW, the resulting algorithm would be:
> 
> tested_flags = page_flags ^ negated_flags;
> if (~tested_flags & required_flags)
>   skip page;
> if (!(tested_flags & anyof_flags))
>   skip_page;
> 
> aggregate_on(page_flags & return_flags);
> 
> Best Regards
> Michał Mirosław

[1] https://lore.kernel.org/all/YyiDg79flhWoMDZB@gmail.com

-- 
BR,
Muhammad Usama Anjum