linux-kernel - Re: [PATCH] kernel/kcov: Replace vm_insert_page with vmf_insert

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Date:   Fri, 21 Sep 2018 14:53:15 +0300
From:   Andrey Ryabinin <aryabinin@...tuozzo.com>
To:     Souptick Joarder <jrdr.linux@...il.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mark Rutland <mark.rutland@....com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Greg KH <gregkh@...uxfoundation.org>, tchibo@...gle.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kernel/kcov: Replace vm_insert_page with vmf_insert_page



On 09/21/2018 01:03 PM, Souptick Joarder wrote:
> On Fri, Sep 21, 2018 at 3:06 PM Andrey Ryabinin <aryabinin@...tuozzo.com> wrote:
>>
>> On 09/20/2018 10:12 PM, Souptick Joarder wrote:
>>> There is a plan to replace vm_insert_page with new API
>>> vmf_insert_page. As part of it, converting vm_insert_page
>>> to use vmf_insert_page.
>>>
>>> Signed-off-by: Souptick Joarder <jrdr.linux@...il.com>
>>> ---
>>>  kernel/kcov.c | 5 +++--
>>>  1 file changed, 3 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/kernel/kcov.c b/kernel/kcov.c
>>> index 3ebd09e..8900d8e 100644
>>> --- a/kernel/kcov.c
>>> +++ b/kernel/kcov.c
>>> @@ -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().

> please refer below commit on linus tree -
> 1c8f422059ae5da07db74

That the commit has nothing to deal with kcov, it doesn't explain why kcov_mmap()
can't keep using vm_insert_page() and/or why we should prefer vmf_insert_page()
over vm_insert_page() particularly in the case of kcov_mmap().