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SubjectRe: [PATCH v2] mm: Introduce new function vm_insert_kmem_page
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.

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