On Thu, Aug 8, 2024 at 1:50 PM Jason Wang <jasow...@redhat.com> wrote: > > On Wed, Aug 7, 2024 at 2:54 PM Yongji Xie <xieyon...@bytedance.com> wrote: > > > > On Wed, Aug 7, 2024 at 12:38 PM Jason Wang <jasow...@redhat.com> wrote: > > > > > > On Wed, Aug 7, 2024 at 11:13 AM Yongji Xie <xieyon...@bytedance.com> > > > wrote: > > > > > > > > On Wed, Aug 7, 2024 at 10:39 AM Jason Wang <jasow...@redhat.com> wrote: > > > > > > > > > > On Tue, Aug 6, 2024 at 11:10 AM Yongji Xie <xieyon...@bytedance.com> > > > > > wrote: > > > > > > > > > > > > On Tue, Aug 6, 2024 at 10:28 AM Jason Wang <jasow...@redhat.com> > > > > > > wrote: > > > > > > > > > > > > > > On Mon, Aug 5, 2024 at 6:42 PM Yongji Xie > > > > > > > <xieyon...@bytedance.com> wrote: > > > > > > > > > > > > > > > > On Mon, Aug 5, 2024 at 4:24 PM Jason Wang <jasow...@redhat.com> > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > On Mon, Aug 5, 2024 at 4:21 PM Jason Wang > > > > > > > > > <jasow...@redhat.com> wrote: > > > > > > > > > > > > > > > > > > > > Barry said [1]: > > > > > > > > > > > > > > > > > > > > """ > > > > > > > > > > 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. > > > > > > > > > > ""“ > > > > > > > > > > > > > > > > > > > > Unfortuantely, we do that under read lock. A possible way > > > > > > > > > > to fix that > > > > > > > > > > is to move the pages allocation out of the lock into the > > > > > > > > > > caller, but > > > > > > > > > > having to allocate a huge number of pages and auxiliary > > > > > > > > > > page array > > > > > > > > > > seems to be problematic as well per Tetsuon [2]: > > > > > > > > > > > > > > > > > > > > """ > > > > > > > > > > You should implement proper error handling instead of using > > > > > > > > > > __GFP_NOFAIL if count can become large. > > > > > > > > > > """ > > > > > > > > > > > > > > > > > > > > > > > > > > I think the problem is it's hard to do the error handling in > > > > > > > > fops->release() currently. > > > > > > > > > > > > > > vduse_dev_dereg_umem() should be the same, it's very hard to > > > > > > > allow it to fail. > > > > > > > > > > > > > > > > > > > > > > > So can we temporarily hold the user page refcount, and release > > > > > > > > it when > > > > > > > > vduse_dev_open()/vduse_domain_release() is executed. The kernel > > > > > > > > page > > > > > > > > allocation and memcpy can be done in vduse_dev_open() which > > > > > > > > allows > > > > > > > > some error handling. > > > > > > > > > > > > > > Just to make sure I understand this, the free is probably not the > > > > > > > big > > > > > > > issue but the allocation itself. > > > > > > > > > > > > > > > > > > > Yes, so defer the allocation might be a solution. > > > > > > > > > > Would you mind posting a patch for this? > > > > > > > > > > > > > > > > > > And if we do the memcpy() in open(), it seems to be a subtle > > > > > > > userspace > > > > > > > noticeable change? (Or I don't get how copying in > > > > > > > vduse_dev_open() can > > > > > > > help here). > > > > > > > > > > > > > > > > > > > Maybe we don't need to do the copy in open(). We can hold the user > > > > > > page refcount until the inflight I/O is completed. That means the > > > > > > allocation of new kernel pages can be done in > > > > > > vduse_domain_map_bounce_page() and the release of old user pages can > > > > > > be done in vduse_domain_unmap_bounce_page(). > > > > > > > > > > This seems to be a subtle userspace noticeable behaviour? > > > > > > > > > > > > > Yes, userspace needs to ensure that it does not reuse the old user > > > > pages for other purposes before vduse_dev_dereg_umem() returns > > > > successfully. The vduse_dev_dereg_umem() will only return successfully > > > > when there is no inflight I/O which means we don't need to allocate > > > > extra kernel pages to store data. If we can't accept this, then your > > > > current patch might be the most suitable. > > > > > > It might be better to not break. > > > > > > Actually during my testing, the read_lock in the do_bounce path slows > > > down the performance. Remove read_lock or use rcu_read_lock() to give > > > 20% improvement of PPS. > > > > > > > Looks like rcu_read_lock() should be OK here. > > The tricky part is that we may still end up behaviour changes (or lose > some of the synchronization between kernel and bounce pages): > > RCU allows the read to be executed in parallel with the writer. So > bouncing could be done in parallel with > vduse_domain_add_user_bounce_pages(), there would be a race in two > memcpy. >
Hmm...this is a problem. We may still need some userspace noticeable behaviour, e.g. only allowing reg_umem/dereg_umem when the device is not started. Thanks, Yongji
