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@gmail.com wrote: [...]
- 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