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. > 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(). Of course, we still have a copy (old user page -> new user spage) if the daemon calls vduse_dev_reg_umem() again. Thanks, Yongji
