Re: [Xen-devel] [PATCH 00/34] put_user_pages(): miscellaneous call sites

[Jan Kara <jack@xxxxxxx>]

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?

                                                                Honza

-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR

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