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

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Message-ID: <CAFqt6za6VJ7vmWEqg4JGWqRiqbfc6GYDnpUeBQ+03xdY51-GQA@mail.gmail.com>
Date:   Fri, 21 Sep 2018 18:55:47 +0530
From:   Souptick Joarder <jrdr.linux@...il.com>
To:     aryabinin@...tuozzo.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 Fri, Sep 21, 2018 at 5:22 PM Andrey Ryabinin <aryabinin@...tuozzo.com> wrote:
>
>
>
> 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().

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.

>
> > 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().
>