Re: [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 1:25 PM Daniel Vetter <daniel.vetter@xxxxxxxx> wrote:
>
> On Wed, Feb 3, 2021 at 9:29 PM Daniel Vetter <daniel.vetter@xxxxxxxx> wrote:
> >
> > On Wed, Feb 3, 2021 at 9:20 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
> > >
> > > On Wed, Feb 3, 2021 at 12:52 AM Daniel Vetter <daniel.vetter@xxxxxxxx> wrote:
> > > >
> > > > On Wed, Feb 3, 2021 at 2:57 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> 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.

Thanks for the input, Daniel! Let me think about the cases you pointed out.

IMHO, the issue with PSS is the difficulty of calculating this metric
without struct page usage. I don't think that problem becomes easier
if we use cgroups or any other API. I wanted to enable existing PSS
calculation mechanisms for the dmabufs known to be backed by struct
pages (since we know how the heap allocated that memory), but sounds
like this would lead to problems that I did not consider.
Thanks,
Suren.

> -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