linux-kernel - Re: [PATCH v2 1/5] vduse: Remove unnecessary spin lock protection

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Message-ID: <CACycT3sNt_PcPQ__KVSqV6xd=+z5+gbMqWhu6H3vcj_fTGzUsw@mail.gmail.com>
Date:   Wed, 13 Jul 2022 19:08:42 +0800
From:   Yongji Xie <xieyongji@...edance.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Liu Xiaodong <xiaodong.liu@...el.com>,
        Maxime Coquelin <maxime.coquelin@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        songmuchun@...edance.com,
        virtualization <virtualization@...ts.linux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/5] vduse: Remove unnecessary spin lock protection

On Wed, Jul 13, 2022 at 1:44 PM Jason Wang <jasowang@...hat.com> wrote:
>
>
> 在 2022/7/6 13:04, Xie Yongji 写道:
> > Taking iotlb lock to access bounce page in page fault
> > handler is meaningless since vduse_domain_free_bounce_pages()
> > would only be called during file release.
> >
> > Signed-off-by: Xie Yongji <xieyongji@...edance.com>
> > ---
> >   drivers/vdpa/vdpa_user/iova_domain.c | 7 ++-----
> >   1 file changed, 2 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/vdpa/vdpa_user/iova_domain.c b/drivers/vdpa/vdpa_user/iova_domain.c
> > index 6daa3978d290..bca1f0b8850c 100644
> > --- a/drivers/vdpa/vdpa_user/iova_domain.c
> > +++ b/drivers/vdpa/vdpa_user/iova_domain.c
> > @@ -211,17 +211,14 @@ static struct page *
> >   vduse_domain_get_bounce_page(struct vduse_iova_domain *domain, u64 iova)
> >   {
> >       struct vduse_bounce_map *map;
> > -     struct page *page = NULL;
> > +     struct page *page;
> >
> > -     spin_lock(&domain->iotlb_lock);
> >       map = &domain->bounce_maps[iova >> PAGE_SHIFT];
> >       if (!map->bounce_page)
> > -             goto out;
> > +             return NULL;
>
>
> Interesting, I wonder why we don't serialize with
> vduse_domain_map_bounce_page() with iotlb_lock?
>

Userspace should only access the bounce page after we set up the dma
mapping, so we don't need serialization from the iotlb_lock in this
case. And vduse_domain_map_bounce_page() only sets the
map->bounce_page rather than clears the map->bounce_page, we would not
have any problem without the lock protection.

Thanks,
Yongji