On Tue 25-02-20 11:09:37, Dave Chinner wrote: > /me wonders if the best thing to do is to add a ->fault callout to > tell the filesystem to lock/unlock the inode right up at the top of > the page fault path, outside even the mmap_sem. That means all the > methods that the page fault calls are protected against S_DAX > changes, and it gives us a low cost method of serialising page > faults against DIO (e.g. via inode_dio_wait()).... Well, that's going to be pretty hard. The main problem is: you cannot lookup VMA until you hold mmap_sem and the inode is inside the VMA. And this is a fundamental problem because until you hold mmap_sem, the address space can change and thus the virtual address you are faulting can be changing inode it is mapped to. So you would have to do some dance like: lock mmap_sem lookup vma get inode reference drop mmap_sem tell fs about page fault lock mmap_sem is the vma still the same? And I'm pretty confident the overhead will be visible in page fault intensive workloads... Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
