On Wed, Aug 7, 2024 at 11:13 AM Yongji Xie <xieyongji@xxxxxxxxxxxxx> wrote: > > On Wed, Aug 7, 2024 at 10:39 AM Jason Wang <jasowang@xxxxxxxxxx> wrote: > > > > On Tue, Aug 6, 2024 at 11:10 AM Yongji Xie <xieyongji@xxxxxxxxxxxxx> wrote: > > > > > > On Tue, Aug 6, 2024 at 10:28 AM Jason Wang <jasowang@xxxxxxxxxx> wrote: > > > > > > > > On Mon, Aug 5, 2024 at 6:42 PM Yongji Xie <xieyongji@xxxxxxxxxxxxx> wrote: > > > > > > > > > > On Mon, Aug 5, 2024 at 4:24 PM Jason Wang <jasowang@xxxxxxxxxx> wrote: > > > > > > > > > > > > On Mon, Aug 5, 2024 at 4:21 PM Jason Wang <jasowang@xxxxxxxxxx> 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. I do want to get rid of it (e.g moving the copying some other where) seems it meets the exact "issue" here which introduces some behaviour change... > I will test this patch > first. > Great. Thanks > Thanks, > Yongji >
