linux-kernel - Re: [v2] fs/proc/task_mmu: Implement IOCTL for efficient page table scanning

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Date:   Tue, 25 Jul 2023 14:09:32 +0500
From:   Muhammad Usama Anjum <usama.anjum@...labora.com>
To:     Michał Mirosław <emmir@...gle.com>
Cc:     Muhammad Usama Anjum <usama.anjum@...labora.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>,
        Mike Rapoport <rppt@...nel.org>, Nadav Amit <namit@...are.com>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Paul Gofman <pgofman@...eweavers.com>,
        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: [v2] fs/proc/task_mmu: Implement IOCTL for efficient page table
 scanning

On 7/25/23 12:23 PM, Muhammad Usama Anjum wrote:
> On 7/24/23 9:10 PM, Michał Mirosław wrote:
> [...]>>>> @@ -2219,22 +2225,24 @@ static int pagemap_scan_get_args(struct
> pm_scan_arg
>>>>> *arg,
>>>>>              arg->category_anyof_mask | arg->return_mask) & ~PM_SCAN_CATEGORIES)
>>>>>                 return -EINVAL;
>>>>>
>>>>> -       start = untagged_addr((unsigned long)arg->start);
>>>>> -       end = untagged_addr((unsigned long)arg->end);
>>>>> -       vec = untagged_addr((unsigned long)arg->vec);
>>>>> +       arg->start = untagged_addr((unsigned long)arg->start);
>>>>> +       arg->end = untagged_addr((unsigned long)arg->end);
>>>>> +       arg->vec = untagged_addr((unsigned long)arg->vec);
>>>>
>>>> BTW, We should we keep the tag in args writeback().
>>> Sorry what?
>>> After this function, the start, end and vec would be used. We need to make
>>> sure that the address are untagged before that.
>>
>> We do write back the address the walk ended at to arg->start in
>> userspace. This pointer I think needs the tag reconstructed so that
>> retrying the ioctl() will be possible.
> Even if we reconstruct the tag for end and vec, We need to update the start
> address. Can we just put same tag as original start in it? I'm not sure.
The special users would use tags. If they are using it, they'll just re-add
the tag in next invocation. I think this implementation is correct.

> 

-- 
BR,
Muhammad Usama Anjum