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

[Russell King - ARM Linux <linux@armlinux.org.uk>]

On Thu, Oct 04, 2018 at 05:45:13PM +0530, Souptick Joarder wrote:
> On Thu, Oct 4, 2018 at 3:45 AM Russell King - ARM Linux
> <linux@armlinux.org.uk> 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.

I'm confused, what are you trying to do?

It seems that we already have:

vm_insert_page() - returns an errno
vmf_insert_page() - returns a VM_FAULT_* code

>From what I _think_ you're saying, you're trying to provide
vm_insert_kmem_page() as a direct replacement for the existing
vm_insert_page(), and then make vm_insert_page() behave as per
vmf_insert_page(), so we end up with:

vm_insert_kmem_page() - returns an errno
vm_insert_page() - returns a VM_FAULT_* code
vmf_insert_page() - returns a VM_FAULT_* code and is identical to
      vm_insert_page()

Given that the documentation for vm_insert_page() says:

 * Usually this function is called from f_op->mmap() handler
 * under mm->mmap_sem write-lock, so it can change vma->vm_flags.
 * Caller must set VM_MIXEDMAP on vma if it wants to call this
 * function from other places, for example from page-fault handler.

this says that the "usual" use method for vm_insert_page() is
_outside_ of page fault handling - if it is used _inside_ page fault
handling, then it states that additional fixups are required on the
VMA.  So I don't get why your patch commentry seems to be saying that
users of vm_insert_page() outside of page fault handling all need to
be patched - isn't this the use case that this function is defined
to be handling?

If you're going to be changing the semantics, doesn't this create a
flag day where we could get new users of vm_insert_page() using the
_existing_ semantics merged after you've changed its semantics (eg,
the return code)?

Maybe I don't understand fully what you're trying to achieve here.

-- 
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