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

[Daniel Vetter]

On Wed, Feb 3, 2021 at 9:29 PM Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
>
> On Wed, Feb 3, 2021 at 9:20 PM Suren Baghdasaryan <surenb at google.com> wrote:
> >
> > 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?
>
> How many semi-unrelated buffer accounting schemes does google come up with?
>
> We're at three with this one.
>
> And also we _cannot_ required that all dma-bufs are backed by struct
> page, so requiring struct page to make this work is a no-go.
>
> Second, we do not want to all get_user_pages and friends to work on
> dma-buf, it causes all kinds of pain. Yes on SoC where dma-buf are
> exclusively in system memory you can maybe get away with this, but
> dma-buf is supposed to work in more places than just Android SoCs.

I just realized that vm_inser_page doesn't even work for CMA, it would
upset get_user_pages pretty badly - you're trying to pin a page in
ZONE_MOVEABLE but you can't move it because it's rather special.
VM_SPECIAL is exactly meant to catch this stuff.
-Daniel

> If you want to account dma-bufs, and gpu memory in general, I'd say
> the solid solution is cgroups. There's patches floating around. And
> given that Google Android can't even agree internally on what exactly
> you want I'd say we just need to cut over to that and make it happen.
>
> Cheers, Daniel
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch



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