On Thu, 31 Jul 2025 07:21:57 -0700, Christoph Hellwig wrote: > On Thu, Jul 32, 2025 at 12:44:09AM +0800, alexjlzheng@xxxxxxxxx wrote: > > From: Jinliang Zheng <alexjlzheng@xxxxxxxxxxx> > > > > Prefaulting the write source buffer incurs an extra userspace access > > in the common fast path. Make iomap_write_iter() consistent with > > generic_perform_write(): only touch userspace an extra time when > > copy_folio_from_iter_atomic() has failed to make progress. > > This is probably a good thing to have, but I'm curous if you did see > it making a different for workloads? Yes, there is some improvement. However, I tested it only a few times, so I can't rule out jitter. However, from a design pattern perspective, this patch is a good thing anyway. > > > + /* > > + * Faults here on mmap()s can recurse into arbitrary > > + * filesystem code. Lots of locks are held that can > > + * deadlock. Use an atomic copy to avoid deadlocking > > + * in page fault handling. > > We can and should use all 80 characters in a line for comments. I agree. hahaha :) thanks, Jinliang Zheng. :) :) > > > + /* > > + * 'folio' is now unlocked and faults on it can be > > + * handled. Ensure forward progress by trying to > > + * fault it in now. > > + */ > > Same here. > > I really wish we could find a way to share the core write loop between > at least iomap and generic_perform_write and maybe also the other copy > and pasters. But that's for another time..
