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

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Feb 2, 2021 at 5:55 PM Matthew Wilcox <willy@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.

We had a discussion in https://lore.kernel.org/patchwork/patch/1372409
about how some DRM drivers set up their VMAs with VM_PFNMAP before
mapping them. We want to use vm_insert_page instead of remap_pfn_range
in the dmabuf heaps so that this memory is visible in PSS. However if
a driver that sets VM_PFNMAP tries to use a dmabuf heap, it will step
into this BUG_ON. We wanted to catch and gradually fix such drivers
but without causing a panic in the process. I hope this clarifies the
reasons why I'm making this change and I'm open to other ideas if they
would address this issue in a better way.