On Fri 02-08-19 07:24:43, Matthew Wilcox wrote: > On Fri, Aug 02, 2019 at 02:41:46PM +0200, Jan Kara wrote: > > On Fri 02-08-19 11:12:44, Michal Hocko wrote: > > > On Thu 01-08-19 19:19:31, john.hubbard@xxxxxxxxx wrote: > > > [...] > > > > 2) Convert all of the call sites for get_user_pages*(), to > > > > invoke put_user_page*(), instead of put_page(). This involves dozens of > > > > call sites, and will take some time. > > > > > > How do we make sure this is the case and it will remain the case in the > > > future? There must be some automagic to enforce/check that. It is simply > > > not manageable to do it every now and then because then 3) will simply > > > be never safe. > > > > > > Have you considered coccinele or some other scripted way to do the > > > transition? I have no idea how to deal with future changes that would > > > break the balance though. > > > > Yeah, that's why I've been suggesting at LSF/MM that we may need to create > > a gup wrapper - say vaddr_pin_pages() - and track which sites dropping > > references got converted by using this wrapper instead of gup. The > > counterpart would then be more logically named as unpin_page() or whatever > > instead of put_user_page(). Sure this is not completely foolproof (you can > > create new callsite using vaddr_pin_pages() and then just drop refs using > > put_page()) but I suppose it would be a high enough barrier for missed > > conversions... Thoughts? > > I think the API we really need is get_user_bvec() / put_user_bvec(), > and I know Christoph has been putting some work into that. That avoids > doing refcount operations on hundreds of pages if the page in question is > a huge page. Once people are switched over to that, they won't be tempted > to manually call put_page() on the individual constituent pages of a bvec. Well, get_user_bvec() is certainly a good API for one class of users but just looking at the above series, you'll see there are *many* places that just don't work with bvecs at all and you need something for those. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
