Re: [PATCH 2/6] i915/gvt/kvm: a NULL ->mm does not mean a thread is a kthread

[Yan Zhao]

On Mon, Apr 13, 2020 at 03:27:30PM +0200, Christoph Hellwig wrote:
> On Mon, Apr 06, 2020 at 11:08:46PM -0400, Yan Zhao wrote:
> > hi
> > we were removing this code. see
> > https://lore.kernel.org/kvm/20200313031109.7989-1-yan.y.zhao@xxxxxxxxx/
> 
> This didn't make 5.7-rc1.
> 
> > The implementation of vfio_dma_rw() has been in vfio next tree.
> > https://github.com/awilliam/linux-vfio/commit/8d46c0cca5f4dc0538173d62cd36b1119b5105bc
> 
> 
> This made 5.7-rc1, so I'll update the series to take it into account.
> 
> T
> > in vfio_dma_rw(),  we still use
> > bool kthread = current->mm == NULL.
> > because if current->mm != NULL and current->flags & PF_KTHREAD, instead
> > of calling use_mm(), we first check if (current->mm == mm) and allow copy_to_user() if it's true.
> > 
> > Do you think it's all right?
> 
> I can't think of another way for a kernel thread to have a mm indeed.
for example, before calling to vfio_dma_rw(), a kernel thread has already
called use_mm(), then its current->mm is not null, and it has flag
PF_KTHREAD.
in this case, we just want to allow the copy_to_user() directly if
current->mm == mm, rather than call another use_mm() again.

do you think it makes sense?

Thanks
Yan

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