Date:   Fri, 28 Sep 2018 17:57:17 +0530
From:   Souptick Joarder <jrdr.linux@...il.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        pasha.tatashin@...cle.com, riel@...hat.com,
        Matthew Wilcox <willy@...radead.org>,
        Minchan Kim <minchan@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Huang, Ying" <ying.huang@...el.com>, ak@...ux.intel.com,
        rppt@...ux.vnet.ibm.com, linux@...inikbrodowski.net,
        Arnd Bergmann <arnd@...db.de>, mcgrof@...nel.org,
        Linux-MM <linux-mm@...ck.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: Introduce new function vm_insert_kmem_page

On Fri, Sep 28, 2018 at 12:02 AM Michal Hocko <mhocko@...nel.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().

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