Re: [PATCH v12 5/6] khugepaged: enable collapse pmd for pte-mapped THP

[Song Liu]

> On Aug 9, 2019, at 9:30 AM, Song Liu <songliubraving@xxxxxx> wrote:
> 
> 
> 
>> On Aug 9, 2019, at 8:24 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
>> 
>> On 08/08, Song Liu wrote:
>>> 
>>>> On Aug 8, 2019, at 9:33 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
>>>> 
>>>>> +	for (i = 0, addr = haddr; i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE) {
>>>>> +		pte_t *pte = pte_offset_map(pmd, addr);
>>>>> +		struct page *page;
>>>>> +
>>>>> +		if (pte_none(*pte))
>>>>> +			continue;
>>>>> +
>>>>> +		page = vm_normal_page(vma, addr, *pte);
>> 
>> just noticed... shouldn't you also check pte_present() before
>> vm_normal_page() ?
> 
> Good catch! Let me fix this. 
> 
>> 
>>>>> +		if (!page || !PageCompound(page))
>>>>> +			return;
>>>>> +
>>>>> +		if (!hpage) {
>>>>> +			hpage = compound_head(page);
>>>> 
>>>> OK,
>>>> 
>>>>> +			if (hpage->mapping != vma->vm_file->f_mapping)
>>>>> +				return;
>>>> 
>>>> is it really possible? May be WARN_ON(hpage->mapping != vm_file->f_mapping)
>>>> makes more sense ?
>>> 
>>> I haven't found code paths lead to this,
>> 
>> Neither me, that is why I asked. I think this should not be possible,
>> but again this is not my area.
>> 
>>> but this is technically possible.
>>> This pmd could contain subpages from different THPs.
>> 
>> Then please explain how this can happen ?
>> 
>>> The __replace_page()
>>> function in uprobes.c creates similar pmd.
>> 
>> No it doesn't,
>> 
>>> Current uprobe code won't really create this problem, because
>>> !PageCompound() check above is sufficient. But it won't be difficult to
>>> modify uprobe code to break this.
>> 
>> I bet it will be a) difficult and b) the very idea to do this would be wrong.
>> 
>>> For this code to be accurate and safe,
>>> I think both this check and the one below are necessary.
>> 
>> I didn't suggest to remove these checks.
>> 
>>> Also, this code
>>> is not on any critical path, so the overhead should be negligible.
>> 
>> I do not care about overhead. But I do care about a poor reader like me
>> who will try to understand this code.
>> 
>> If you too do not understand how a THP page can have a different mapping
>> then use VM_WARN or at least add a comment to explain that this is not
>> supposed to happen!
> 
> Fair enough. I will add WARN and more comments. 
> 
> Thanks,
> Song

To reduce spamming, I attached updated 5/6 here. 

Thanks,
Song

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