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Date:   Tue, 14 Apr 2020 09:00:13 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Yan Zhao <yan.y.zhao@intel.com>
Cc:     Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
        Felipe Balbi <balbi@kernel.org>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        intel-gvt-dev@lists.freedesktop.org,
        Felix Kuehling <Felix.Kuehling@amd.com>,
        linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org,
        amd-gfx@lists.freedesktop.org, io-uring@vger.kernel.org,
        linux-mm@kvack.org, Zhenyu Wang <zhenyuw@linux.intel.com>,
        intel-gfx@lists.freedesktop.org, linux-fsdevel@vger.kernel.org,
        Alex Deucher <alexander.deucher@amd.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        virtualization@lists.linux-foundation.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Zhi Wang <zhi.a.wang@intel.com>,
        Al Viro <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH 2/6] i915/gvt/kvm: a NULL ->mm does not mean a thread
 is a kthread
Message-ID: <20200414070013.GA23680@lst.de>
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On Mon, Apr 13, 2020 at 08:04:10PM -0400, Yan Zhao wrote:
> > 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?

I mean no other way than using use_mm.  That being said nesting
potentional use_mm callers sounds like a rather bad idea, and we
should avoid that.