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Subject: Re: [PATCH] kernel/kcov: Replace vm_insert_page with vmf_insert_page
To:     Souptick Joarder <jrdr.linux@gmail.com>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Greg KH <gregkh@linuxfoundation.org>, tchibo@google.com,
        linux-kernel@vger.kernel.org
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From:   Andrey Ryabinin <aryabinin@virtuozzo.com>
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Date:   Fri, 21 Sep 2018 17:45:15 +0300
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On 09/21/2018 04:25 PM, Souptick Joarder wrote:
> On Fri, Sep 21, 2018 at 5:22 PM Andrey Ryabinin <aryabinin@virtuozzo.com> wrote:
>> On 09/21/2018 01:03 PM, Souptick Joarder wrote:
>>> On Fri, Sep 21, 2018 at 3:06 PM Andrey Ryabinin <aryabinin@virtuozzo.com> wrote:
>>>>
>>>> On 09/20/2018 10:12 PM, Souptick Joarder wrote:
>>>>> @@ -293,8 +293,9 @@ static int kcov_mmap(struct file *filep, struct vm_area_struct *vma)
>>>>>               spin_unlock(&kcov->lock);
>>>>>               for (off = 0; off < size; off += PAGE_SIZE) {
>>>>>                       page = vmalloc_to_page(kcov->area + off);
>>>>> -                     if (vm_insert_page(vma, vma->vm_start + off, page))
>>>>> -                             WARN_ONCE(1, "vm_insert_page() failed");
>>>>> +                     if (vmf_insert_page(vma, vma->vm_start + off, page)
>>>>> +                                     != VM_FAULT_NOPAGE)
>>>>> +                             WARN_ONCE(1, "vmf_insert_page() failed");
>>>>
>>>> Nack, don't see the reason for such change, it only makes code worse.
>>>
>>> Yes, it needed.
>>
>> No, it's not needed. vm_insert_page() works perfectly fine here.
>>
>>> Going forward vm_insert_page will be converted to
>>> vmf_insert_page. As part of it, this code has to be converted to use
>>> vmf_insert_page().
>>
>> This doesn't explain why such conversion would make sense for kcov_mmap().
> 
> vm_insert_page used to return errno which individual drivers have to
> map to VM_FAULT_CODE.
> There were also places where return value of vm_insert_page was
> ignored and return a
> TRUE value.
> 
> As part of vm_fault_t migration patches, we have identified and clean
> up all these.
> The plan was to introduce inline wrapper vmf_insert_page(), convert all
> the page fault handlers to use it and then convert vm_insert_page to
> vmf_insert_page. By now we have converted all the page fault handlers
> to use this API.
> 
> Still their are few places where vm_insert_page is called outside fault handlers
> context to map kernel page to user vma and kernel/kcov.c is one of them.
> 
> Now removing vm_insert_page from those few places one by one.
> 
> Going forward vm_insert_page will be removed from kernel permanently,
> so that new drivers can't use this API and create new errno to VM_FAULT_CODE
> mapping code.
> 

So, to avoid addition of new code that converts errno->VM_FAULT_* you propose to add
new code that does the opposite - converts VM_FAULT_* to errno? How is that better?

And what function is going to be next? pte_none()? It is also used in fault handlers.
Are there going to be vmf_pte_none() and complete removal of pte_none() "so that new drivers
can't use pte_none() and create new pte_none() to VM_FAULT_CODE mapping code"?

Don't you see that how that doesn't make any sense?
Want vmf_insert_page() for easier error handling in fault handlers? - fine, use it in fault handlers.
But leave the vm_insert_page() for the code that has nothing to do with fault handling.