On Tue, Feb 2, 2021 at 5:55 PM 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. 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.
