[Linaro-mm-sig] [PATCH 1/2] mm: replace BUG_ON in vm_insert_page with a return of an error

[Suren Baghdasaryan]

On Wed, Feb 3, 2021 at 12:52 AM Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
>
> On Wed, Feb 3, 2021 at 2:57 AM Matthew Wilcox <willy at infradead.org> wrote:
> >
> > On Tue, Feb 02, 2021 at 04:31:33PM -0800, Suren Baghdasaryan wrote:
> > > Replace BUG_ON(vma->vm_flags & VM_PFNMAP) in vm_insert_page with
> > > WARN_ON_ONCE and returning an error. This is to ensure users of the
> > > vm_insert_page that set VM_PFNMAP are notified of the wrong flag usage
> > > and get an indication of an error without panicing the kernel.
> > > This will help identifying drivers that need to clear VM_PFNMAP before
> > > using dmabuf system heap which is moving to use vm_insert_page.
> >
> > NACK.
> >
> > The system may not _panic_, but it is clearly now _broken_.  The device
> > doesn't work, and so the system is useless.  You haven't really improved
> > anything here.  Just bloated the kernel with yet another _ONCE variable
> > that in a normal system will never ever ever be triggered.
>
> Also, what the heck are you doing with your drivers? dma-buf mmap must
> call dma_buf_mmap(), even for forwarded/redirected mmaps from driver
> char nodes. If that doesn't work we have some issues with the calling
> contract for that function, not in vm_insert_page.

The particular issue I observed (details were posted in
https://lore.kernel.org/patchwork/patch/1372409) is that DRM drivers
set VM_PFNMAP flag (via a call to drm_gem_mmap_obj) before calling
dma_buf_mmap. Some drivers clear that flag but some don't. I could not
find the answer to why VM_PFNMAP is required for dmabuf mappings and
maybe someone can explain that here?
If there is a reason to set this flag other than historical use of
carveout memory then we wanted to catch such cases and fix the drivers
that moved to using dmabuf heaps. However maybe there are other
reasons and if so I would be very grateful if someone could explain
them. That would help me to come up with a better solution.

> Finally why exactly do we need to make this switch for system heap?
> I've recently looked at gup usage by random drivers, and found a lot
> of worrying things there. gup on dma-buf is really bad idea in
> general.

The reason for the switch is to be able to account dmabufs allocated
using dmabuf heaps to the processes that map them. The next patch in
this series https://lore.kernel.org/patchwork/patch/1374851
implementing the switch contains more details and there is an active
discussion there. Would you mind joining that discussion to keep it in
one place?
Thanks!

> -Daniel
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch