> 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
