Re: [PATCH RFC 1/5] vpda: try to fix the potential crash due to misusing __GFP_NOFAIL

Barry Song
2024-07-24T22:51:00Z

On Thu, Jul 25, 2024 at 12:27 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Wed 24-07-24 20:55:40, Barry Song wrote:
> > From: Barry Song <v-songbaohua@oppo.com>
> >
> > mm doesn't support non-blockable __GFP_NOFAIL allocation. Because
> > __GFP_NOFAIL without direct reclamation may just result in a busy
> > loop within non-sleepable contexts.
> >
> > static inline struct page *
> > __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> > struct alloc_context *ac)
> > {
> > ...
> > /*
> > * Make sure that __GFP_NOFAIL request doesn't leak out and make sure
> > * we always retry
> > */
> > if (gfp_mask & __GFP_NOFAIL) {
> > /*
> > * All existing users of the __GFP_NOFAIL are blockable, so warn
> > * of any new users that actually require GFP_NOWAIT
> > */
> > if (WARN_ON_ONCE_GFP(!can_direct_reclaim, gfp_mask))
> > goto fail;
> > ...
> > }
> > ...
> > fail:
> > warn_alloc(gfp_mask, ac->nodemask,
> > "page allocation failure: order:%u", order);
> > got_pg:
> > return page;
> > }
> >
> > Let's move the memory allocation out of the atomic context and use
> > the normal sleepable context to get pages.
> >
> > [RFC]: This has only been compile-tested; I'd prefer if the VDPA maintainers
> > handles it.
> >
> > Cc: "Michael S. Tsirkin" <mst@redhat.com>
> > Cc: Jason Wang <jasowang@redhat.com>
> > Cc: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> > Cc: "Eugenio Pérez" <eperezma@redhat.com>
> > Cc: Maxime Coquelin <maxime.coquelin@redhat.com>
> > Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> > ---
> > drivers/vdpa/vdpa_user/iova_domain.c | 24 ++++++++++++++++++++----
> > 1 file changed, 20 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/vdpa/vdpa_user/iova_domain.c b/drivers/vdpa/vdpa_user/iova_domain.c
> > index 791d38d6284c..eff700e5f7a2 100644
> > --- a/drivers/vdpa/vdpa_user/iova_domain.c
> > +++ b/drivers/vdpa/vdpa_user/iova_domain.c
> > @@ -287,28 +287,44 @@ void vduse_domain_remove_user_bounce_pages(struct vduse_iova_domain *domain)
> > {
> > struct vduse_bounce_map *map;
> > unsigned long i, count;
> > + struct page **pages = NULL;
> >
> > write_lock(&domain->bounce_lock);
> > if (!domain->user_bounce_pages)
> > goto out;
> > -
> > count = domain->bounce_size >> PAGE_SHIFT;
> > + write_unlock(&domain->bounce_lock);
> > +
> > + pages = kmalloc_array(count, sizeof(*pages), GFP_KERNEL | __GFP_NOFAIL);
> > + for (i = 0; i < count; i++)
> > + pages[i] = alloc_page(GFP_KERNEL | __GFP_NOFAIL);
>
> AFAICS vduse_domain_release calls this function with
> spin_lock(&domain->iotlb_lock) so dropping &domain->bounce_lock is not
> sufficient.

yes. this is true:

static int vduse_domain_release(struct inode *inode, struct file *file)
{
struct vduse_iova_domain *domain = file->private_data;

spin_lock(&domain->iotlb_lock);
vduse_iotlb_del_range(domain, 0, ULLONG_MAX);
vduse_domain_remove_user_bounce_pages(domain);
vduse_domain_free_kernel_bounce_pages(domain);
spin_unlock(&domain->iotlb_lock);
put_iova_domain(&domain->stream_iovad);
put_iova_domain(&domain->consistent_iovad);
vhost_iotlb_free(domain->iotlb);
vfree(domain->bounce_maps);
kfree(domain);

return 0;
}

This is quite a pain. I admit I don't have knowledge of this driver, and I don't
think it's safe to release two locks and then reacquire them. The situation is
rather complex. Therefore, I would prefer if the VDPA maintainers could
take the lead in implementing a proper fix.

>
> --
> Michal Hocko
> SUSE Labs

Thanks
Barry