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: <20200414070344.GF10586@joy-OptiPlex-7040>
Date:   Tue, 14 Apr 2020 03:03:44 -0400
From:   Yan Zhao <yan.y.zhao@...el.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Jens Axboe <axboe@...nel.dk>, Felipe Balbi <balbi@...nel.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        intel-gvt-dev@...ts.freedesktop.org,
        Felix Kuehling <Felix.Kuehling@....com>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        amd-gfx@...ts.freedesktop.org, io-uring@...r.kernel.org,
        linux-mm@...ck.org, Zhenyu Wang <zhenyuw@...ux.intel.com>,
        intel-gfx@...ts.freedesktop.org, linux-fsdevel@...r.kernel.org,
        Alex Deucher <alexander.deucher@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        virtualization@...ts.linux-foundation.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Zhi Wang <zhi.a.wang@...el.com>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH 2/6] i915/gvt/kvm: a NULL ->mm does not mean a thread is
 a kthread

On Tue, Apr 14, 2020 at 09:00:13AM +0200, Christoph Hellwig wrote:
> 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.
yes, agree.
I was explaining why we just use "current->mm == NULL"
(not "current->flag & PF_KTHREAD") as a criteria to call use_mm()
in vfio_dma_rw(), which you might ask us when you take that part into your
series. :)