linux-kernel - Re: [Linaro-mm-sig] [PATCH 1/2] mm: replace BUG_ON in vm_insert

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Date:   Wed, 3 Feb 2021 09:52:22 +0100
From:   Daniel Vetter <daniel.vetter@...ll.ch>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Suren Baghdasaryan <surenb@...gle.com>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>,
        Sandeep Patil <sspatil@...gle.com>,
        Christian König <christian.koenig@....com>,
        Android Kernel Team <kernel-team@...roid.com>,
        James Jones <jajones@...dia.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Liam Mark <lmark@...eaurora.org>,
        Brian Starkey <Brian.Starkey@....com>,
        Christoph Hellwig <hch@...radead.org>,
        Minchan Kim <minchan@...nel.org>,
        Linux MM <linux-mm@...ck.org>,
        John Stultz <john.stultz@...aro.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Chris Goldsworthy <cgoldswo@...eaurora.org>,
        Hridya Valsaraju <hridya@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Robin Murphy <robin.murphy@....com>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>
Subject: Re: [Linaro-mm-sig] [PATCH 1/2] mm: replace BUG_ON in vm_insert_page
 with a return of an error

On Wed, Feb 3, 2021 at 2:57 AM Matthew Wilcox <willy@...radead.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.

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