Re: [PATCH 1/1] dma-buf: heaps: Map system heap pages as managed by linux vm

[Suren Baghdasaryan]

On Tue, Feb 2, 2021 at 12:51 AM Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
>
> On Tue, Feb 02, 2021 at 12:44:44AM -0800, Suren Baghdasaryan wrote:
> > On Mon, Feb 1, 2021 at 11:03 PM Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
> > >
> > > IMHO the
> > >
> > >         BUG_ON(vma->vm_flags & VM_PFNMAP);
> > >
> > > in vm_insert_page should just become a WARN_ON_ONCE with an error
> > > return, and then we just need to gradually fix up the callers that
> > > trigger it instead of coming up with workarounds like this.
> >
> > For the existing vm_insert_page users this should be fine since
> > BUG_ON() guarantees that none of them sets VM_PFNMAP.
>
> Even for them WARN_ON_ONCE plus an actual error return is a way
> better assert that is much developer friendly.

Agree.

>
> > However, for the
> > system_heap_mmap I have one concern. When vm_insert_page returns an
> > error due to VM_PFNMAP flag, the whole mmap operation should fail
> > (system_heap_mmap returning an error leading to dma_buf_mmap failure).
> > Could there be cases when a heap user (DRM driver for example) would
> > be expected to work with a heap which requires VM_PFNMAP and at the
> > same time with another heap which requires !VM_PFNMAP? IOW, this
> > introduces a dependency between the heap and its
> > user. The user would have to know expectations of the heap it uses and
> > can't work with another heap that has the opposite expectation. This
> > usecase is purely theoretical and maybe I should not worry about it
> > for now?
>
> If such a case ever arises we can look into it.

Sounds good. I'll prepare a new patch and will post it later today. Thanks!