linux-kernel - Re: [PATCH v21 2/5] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs

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Date:   Wed, 28 Jun 2023 11:03:46 +0500
From:   Muhammad Usama Anjum <usama.anjum@...labora.com>
To:     Michał Mirosław <emmir@...gle.com>,
        Andrei Vagin <avagin@...il.com>
Cc:     Muhammad Usama Anjum <usama.anjum@...labora.com>,
        Peter Xu <peterx@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Danylo Mocherniuk <mdanylo@...gle.com>,
        Paul Gofman <pgofman@...eweavers.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Mike Rapoport <rppt@...nel.org>, Nadav Amit <namit@...are.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>,
        Matthew Wilcox <willy@...radead.org>,
        Pasha Tatashin <pasha.tatashin@...een.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
Subject: Re: [PATCH v21 2/5] fs/proc/task_mmu: Implement IOCTL to get and
 optionally clear info about PTEs

On 6/28/23 12:54 AM, Michał Mirosław wrote:
> On Tue, 27 Jun 2023 at 21:20, Muhammad Usama Anjum
> <usama.anjum@...labora.com> wrote:
>> Thanks Michał for replying.
>>
>> On 6/27/23 11:52 PM, Michał Mirosław wrote:
>>> On Tue, 27 Jun 2023 at 11:00, Muhammad Usama Anjum
>>> <usama.anjum@...labora.com> wrote:
>>>>
>>>> Hi Andrei and Michal,
>>>>
>>>> Lets resolve last two points. Please reply below.
>>>>
>>>> On 6/27/23 6:46 AM, Andrei Vagin wrote:
>>> [...]
>>>>> And we need to report an address where it stopped scanning.
>>>>> We can do that by adding zero length vector.
>>>> I don't want to do multiplexing the ending address in vec. Can we add
>>>> end_addr variable in struct pm_scan_arg to always return the ending address?
>>>>
>>>> struct pm_scan_arg {
>>>>         ...
>>>>         _u64 end_addr;
>>>> };
>>>
>>> The idea to emit a zero-length entry for the end looks nice. This has
>>> the disadvantage that we'd need to either reserve one entry for the
>>> ending marker or stop the walk after the last entry is no longer
>>> matching.
>> This is ambiguous.
> 
> Can you explain? Both solutions would allow to return the restart
> point back to the caller (the second one would need to stop the walk
> as soon as the matching page range finishes -- that creates
> discontinuity).
> 
>>> Another solution would be to rewrite 'start' and 'len'. The caller
>>> would be forced to use non-const `pm_scan_arg`, but I expect the `vec`
>>> pointer would normally be written anyway (unless using only a
>>> statically-allocated buffer).
>>> Also, if the 'len' is replaced with 'end' that would make the ioctl
>>> easily restartable (just call again if start != end).
>> Nice idea. But returning ending address in len seems a bit strange.
> 
> I mean that it would update `start` = start value for next call' and
> `len` = `len` - (new `start` - original `start`).
> 
> By replacing `len` I meant to remove the field and add `end` instead
> to make the requested range use begin .. end (iterator range) style
> instead of start + len (buffer and length). In this version you only
> need to update `start` (or `begin` if you prefer).
The `start` and `end` with updating the `start` with ending address seems
most appropriate. I'll make updates.

> 
> Best Regards
> Michał Mirosław

-- 
BR,
Muhammad Usama Anjum