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Message-ID: <df71c1ca-0f3f-fffb-a5d8-14ea99577387@collabora.com>
Date: Thu, 27 Jul 2023 16:31:18 +0500
From: Muhammad Usama Anjum <usama.anjum@...labora.com>
To: Michał Mirosław <emmir@...gle.com>
Cc: Muhammad Usama Anjum <usama.anjum@...labora.com>,
"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
Michał Mirosław <mirq-linux@...e.qmqm.pl>,
Andrei Vagin <avagin@...il.com>,
Danylo Mocherniuk <mdanylo@...gle.com>,
Alex Sierra <alex.sierra@....com>,
Alexander Viro <viro@...iv.linux.org.uk>,
Andrew Morton <akpm@...ux-foundation.org>,
Axel Rasmussen <axelrasmussen@...gle.com>,
Christian Brauner <brauner@...nel.org>,
Cyrill Gorcunov <gorcunov@...il.com>,
Dan Williams <dan.j.williams@...el.com>,
David Hildenbrand <david@...hat.com>,
Greg KH <gregkh@...uxfoundation.org>,
"Gustavo A . R . Silva" <gustavoars@...nel.org>,
"Liam R . Howlett" <Liam.Howlett@...cle.com>,
Matthew Wilcox <willy@...radead.org>,
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Pasha Tatashin <pasha.tatashin@...een.com>,
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Peter Xu <peterx@...hat.com>, Shuah Khan <shuah@...nel.org>,
Suren Baghdasaryan <surenb@...gle.com>,
Vlastimil Babka <vbabka@...e.cz>,
Yang Shi <shy828301@...il.com>,
Yun Zhou <yun.zhou@...driver.com>,
linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
kernel@...labora.com
Subject: Re: [v3] fs/proc/task_mmu: Implement IOCTL for efficient page table
scanning
On 7/27/23 4:26 PM, Michał Mirosław wrote:
> On Thu, 27 Jul 2023 at 10:03, Muhammad Usama Anjum
> <usama.anjum@...labora.com> wrote:
>> On 7/27/23 2:10 AM, Michał Mirosław wrote:
>>> On Wed, 26 Jul 2023 at 10:34, Muhammad Usama Anjum
>>> <usama.anjum@...labora.com> wrote:
>>>> On 7/25/23 11:05 PM, Michał Mirosław wrote:
>>>>> On Tue, 25 Jul 2023 at 11:11, Muhammad Usama Anjum
>>>>> <usama.anjum@...labora.com> wrote:
> [...]
>>>>> 2. For the address tagging part I'd prefer someone who knows how this
>>>>> is used take a look. We're ignoring the tag (but clear it on return in
>>>>> ->start) - so it doesn't matter for the ioctl() itself.
>>>> I've added Kirill if he can give his thoughts about tagged memory.
>>>>
>>>> Right now we are removing the tags from all 3 pointers (start, end, vec)
>>>> before using the pointers on kernel side. But we are overwriting and
>>>> writing the walk ending address in start which user can read/use.
>>>>
>>>> I think we shouldn't over-write the start (and its tag) and instead return
>>>> the ending walk address in new variable, walk_end.
>>>
>>> The overwrite of `start` is making the ioctl restart (continuation)
>>> easier to handle. I prefer the current way, but it's not a strong
>>> opinion.
>> We shouldn't overwrite the start if we aren't gonna put the correct tag. So
>> I've resorted to adding another variable `walk_end` to return the walk
>> ending address.
>
> Yes. We have two options:
>
> 1. add new field and have the userspace check it and update start
> itself to continue the scan,
I've selected this option and sent v26 already:
https://lore.kernel.org/all/20230727093637.1262110-1-usama.anjum@collabora.com
> or:
> 2. reconstruct the tag from either orignal `start` or `end` and have
> the userspace re-set `start` if it wants to restart the scan instead
> of continuing.
In some case, compiler can put integrity checking metadata in the pointer's
upper byte. So copying start or end's meta data would be wrong.
>
> (the second one, using `end`'s tag, might be the easiest for
> userspace, as it can check `start` == `end` when deciding to continue
> or restart).
>
> Best Regards
> Michał Mirosław
--
BR,
Muhammad Usama Anjum
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