Re: [PATCH] mm: Introduce new function vm_insert_kmem_page

[Souptick Joarder <jrdr.linux@gmail.com>]

Hi Michal,

On Fri, Sep 28, 2018 at 5:57 PM Souptick Joarder <jrdr.linux@gmail.com> wrote:
>
> On Fri, Sep 28, 2018 at 12:02 AM Michal Hocko <mhocko@kernel.org> wrote:
> >
> > On Thu 27-09-18 23:21:23, Souptick Joarder wrote:
> > > vm_insert_kmem_page is similar to vm_insert_page and will
> > > be used by drivers to map kernel (kmalloc/vmalloc/pages)
> > > allocated memory to user vma.
> > >
> > > Previously vm_insert_page is used for both page fault
> > > handlers and outside page fault handlers context. When
> > > vm_insert_page is used in page fault handlers context,
> > > each driver have to map errno to VM_FAULT_CODE in their
> > > own way. But as part of vm_fault_t migration all the
> > > page fault handlers are cleaned up by using new vmf_insert_page.
> > > Going forward, vm_insert_page will be removed by converting
> > > it to vmf_insert_page.
> > >
> > > But their are places where vm_insert_page is used outside
> > > page fault handlers context and converting those to
> > > vmf_insert_page is not a good approach as drivers will end
> > > up with new VM_FAULT_CODE to errno conversion code and it will
> > > make each user more complex.
> > >
> > > So this new vm_insert_kmem_page can be used to map kernel
> > > memory to user vma outside page fault handler context.
> > >
> > > 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.
> > >
> > > We will slowly convert all the user of vm_insert_page to
> > > vm_insert_kmem_page after this API be available in linus tree.
> >
> > In general I do not like patches adding a new exports/functionality
> > without any user added at the same time. I am not going to look at the
> > implementation right now but the above opens more questions than it
> > gives answers. Why do we have to distinguish #PF from other paths?
>
> Going forward, the plan is to restrict future drivers not to use vm_insert_page
> ( *it will generate new errno to VM_FAULT_CODE mapping code for new drivers
> which were already cleaned up for existing drivers*) in #PF context but to make
> use of vmf_insert_page which returns VMF_FAULT_CODE and that is not possible
> until both vm_insert_page and vmf_insert_page API exists.
>
> But there are some consumers of vm_insert_page which use it outside #PF context.
> straight forward conversion of vm_insert_page to vmf_insert_page won't
> work there as
> those function calls expects errno not vm_fault_t in return.
>
> e.g - drivers/auxdisplay/cfag12864bfb.c, line 55
>         drivers/auxdisplay/ht16k33.c, line 227
>         drivers/firewire/core-iso.c, line 115
>         drivers/gpu/drm/rockchip/rockchip_drm_gem.c, line 237
>         drivers/gpu/drm/xen/xen_drm_front_gem.c, line 253
>         drivers/iommu/dma-iommu.c, line 600
>         drivers/media/common/videobuf2/videobuf2-dma-sg.c, line 343
>         drivers/media/usb/usbvision/usbvision-video.c, line 1056
>         drivers/xen/gntalloc.c, line 548
>         drivers/xen/gntdev.c, line 1149
>         drivers/xen/privcmd-buf.c, line 184
>         mm/vmalloc.c, line 2254
>         net/ipv4/tcp.c, line 1806
>         net/packet/af_packet.c, line 4407
>
> These are the approaches which could have been taken to handle this scenario -
>
> 1. 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)
>
> 2. 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.
>
> 3. 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().

Does this 3rd approach looks good or their is a better way to handle
this scenario ?
Remaining vm_fault_t migration work has dependency on this patch.