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Date:   Wed, 28 Jun 2023 11:03:46 +0500
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Cc:     Muhammad Usama Anjum <usama.anjum@collabora.com>,
        Peter Xu <peterx@redhat.com>,
        David Hildenbrand <david@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
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        Mike Rapoport <rppt@kernel.org>, Nadav Amit <namit@vmware.com>,
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        Shuah Khan <shuah@kernel.org>,
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        Vlastimil Babka <vbabka@suse.cz>,
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        Alex Sierra <alex.sierra@amd.com>,
        Matthew Wilcox <willy@infradead.org>,
        Pasha Tatashin <pasha.tatashin@soleen.com>,
        Axel Rasmussen <axelrasmussen@google.com>,
        "Gustavo A . R . Silva" <gustavoars@kernel.org>,
        Dan Williams <dan.j.williams@intel.com>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-mm@kvack.org, linux-kselftest@vger.kernel.org,
        Greg KH <gregkh@linuxfoundation.org>, kernel@collabora.com
Subject: Re: [PATCH v21 2/5] fs/proc/task_mmu: Implement IOCTL to get and
 optionally clear info about PTEs
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        Andrei Vagin <avagin@gmail.com>
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From:   Muhammad Usama Anjum <usama.anjum@collabora.com>
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On 6/28/23 12:54 AM, Michał Mirosław wrote:
> On Tue, 27 Jun 2023 at 21:20, Muhammad Usama Anjum
> <usama.anjum@collabora.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@collabora.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