linux-kernel - Re: [PATCH v2] mm: Introduce new function vm_insert_kmem

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Message-ID: <20181004003956.GA31522@bombadil.infradead.org>
Date:   Wed, 3 Oct 2018 17:39:56 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     Souptick Joarder <jrdr.linux@...il.com>,
        miguel.ojeda.sandonis@...il.com, robin@...tonic.nl,
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Subject: Re: [PATCH v2] mm: Introduce new function vm_insert_kmem_page

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.