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

[Muhammad Usama Anjum]

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