Re: [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 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.

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