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

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Message-ID: <20200413132730.GB14455@lst.de>
Date:   Mon, 13 Apr 2020 15:27:30 +0200
From:   Christoph Hellwig <hch@....de>
To:     Yan Zhao <yan.y.zhao@...el.com>
Cc:     Christoph Hellwig <hch@....de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>, Felipe Balbi <balbi@...nel.org>,
        amd-gfx@...ts.freedesktop.org,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        linux-usb@...r.kernel.org, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Zhenyu Wang <zhenyuw@...ux.intel.com>,
        virtualization@...ts.linux-foundation.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>,
        intel-gfx@...ts.freedesktop.org,
        Alex Deucher <alexander.deucher@....com>,
        intel-gvt-dev@...ts.freedesktop.org,
        Jason Wang <jasowang@...hat.com>,
        Zhi Wang <zhi.a.wang@...el.com>
Subject: Re: [PATCH 2/6] i915/gvt/kvm: a NULL ->mm does not mean a thread
 is a kthread

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@intel.com/

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.