On Wed, Jun 12, 2019 at 12:12:34AM -0700, Christoph Hellwig wrote: > On Tue, Jun 11, 2019 at 04:44:31PM -0300, Jason Gunthorpe wrote: > > On Sat, Jun 08, 2019 at 01:54:25AM -0700, Christoph Hellwig wrote: > > > FYI, I very much disagree with the direction this is moving. > > > > > > struct hmm_mirror literally is a trivial duplication of the > > > mmu_notifiers. All these drivers should just use the mmu_notifiers > > > directly for the mirroring part instead of building a thing wrapper > > > that adds nothing but helping to manage the lifetime of struct hmm, > > > which shouldn't exist to start with. > > > > Christoph: What do you think about this sketch below? > > > > It would replace the hmm_range/mirror/etc with a different way to > > build the same locking scheme using some optional helpers linked to > > the mmu notifier? > > > > (just a sketch, still needs a lot more thinking) > > I like the idea. A few nitpicks: Can we avoid having to store the > mm in struct mmu_notifier? I think we could just easily pass it as a > parameter to the helpers. Yes, but I think any driver that needs to use this API will have to hold the 'struct mm_struct' and the 'struct mmu_notifier' together (ie ODP does this in ib_ucontext_per_mm), so if we put it in the notifier then it is trivially available everwhere it is needed, and the mmu_notifier code takes care of the lifetime for the driver. > The write lock case of mm_invlock_start_write_and_lock is probably > worth factoring into separate helper? I can see cases where drivers > want to just use it directly if they need to force getting the lock > without the chance of a long wait. The entire purpose of the invlock is to avoid getting the write lock on mmap_sem as a fast path - if the driver wishes to use mmap_sem locking only then it should just do so directly and forget about the invlock. Note that this patch is just an API sketch, I haven't fully checked that the range_start/end are actually always called under mmap_sem, and I already found that release is not. So there will need to be some preperatory adjustments before we can use down_write(mmap_sem) as a locking strategy here. Thanks, Jason
