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

Matthew Wilcox
2018-10-04T00:39:56Z

On Wed, Oct 03, 2018 at 11:14:45PM +0100, Russell King - ARM Linux 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:
[...]
> > 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.

Right, but we already did that. We now have vmf_insert_page() which
returns a VM_FAULT_* code and vm_insert_page() which returns an errno.