Re: [PATCH v2] mm: Introduce new function vm_insert_kmem_page

[Souptick Joarder]

On Thu, Oct 4, 2018 at 3:45 AM Russell King - ARM Linux
<linux@xxxxxxxxxxxxxxx> wrote:
>
> On Wed, Oct 03, 2018 at 01:00:03PM -0700, Matthew Wilcox wrote:
> > On Thu, Oct 04, 2018 at 12:28:54AM +0530, Souptick Joarder wrote:
> > > These are the approaches which could have been taken to handle
> > > this scenario -
> > >
> > > *  Replace vm_insert_page with vmf_insert_page and then write few
> > >    extra lines of code to convert VM_FAULT_CODE to errno which
> > >    makes driver users more complex ( also the reverse mapping errno to
> > >    VM_FAULT_CODE have been cleaned up as part of vm_fault_t migration ,
> > >    not preferred to introduce anything similar again)
> > >
> > > *  Maintain both vm_insert_page and vmf_insert_page and use it in
> > >    respective places. But it won't gurantee that vm_insert_page will
> > >    never be used in #PF context.
> > >
> > > *  Introduce a similar API like vm_insert_page, convert all non #PF
> > >    consumer to use it and finally remove vm_insert_page by converting
> > >    it to vmf_insert_page.
> > >
> > > And the 3rd approach was taken by introducing vm_insert_kmem_page().
> > >
> > > In short, vmf_insert_page will be used in page fault handlers
> > > context and vm_insert_kmem_page will be used to map kernel
> > > memory to user vma outside page fault handlers context.
> >
> > As far as I can tell, vm_insert_kmem_page() is line-for-line identical
> > with vm_insert_page().  Seriously, here's a diff I just did:
> >
> > -static int insert_page(struct vm_area_struct *vma, unsigned long addr,
> > -                       struct page *page, pgprot_t prot)
> > +static int insert_kmem_page(struct vm_area_struct *vma, unsigned long addr,
> > +               struct page *page, pgprot_t prot)
> > -       /* Ok, finally just insert the thing.. */
> > -int vm_insert_page(struct vm_area_struct *vma, unsigned long addr,
> > +int vm_insert_kmem_page(struct vm_area_struct *vma, unsigned long addr,
> > -       return insert_page(vma, addr, page, vma->vm_page_prot);
> > +       return insert_kmem_page(vma, addr, page, vma->vm_page_prot);
> > -EXPORT_SYMBOL(vm_insert_page);
> > +EXPORT_SYMBOL(vm_insert_kmem_page);
> >
> > What on earth are you trying to do?

>
> Reading the commit log, it seems that the intention is to split out
> vm_insert_page() used outside of page-fault handling with the use
> within page-fault handling, so that different return codes can be
> used.
>
> I don't see that justifies the code duplication - can't
> vm_insert_page() and vm_insert_kmem_page() use the same mechanics
> to do their job, and just translate the error code from the most-
> specific to the least-specific error code?  Do we really need two
> copies of the same code just to return different error codes.

Sorry about it.
can I take below approach in a patch series ->

create a wrapper function vm_insert_kmem_page using vm_insert_page.
Convert all the non #PF users to use it.
Then make vm_insert_page static and convert inline vmf_insert_page to caller.