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

[Michal Hocko <mhocko@xxxxxxxxxx>]

On Wed 07-08-19 10:37:26, Jan Kara wrote:
> On Fri 02-08-19 12:14:09, John Hubbard wrote:
> > On 8/2/19 7:52 AM, Jan Kara wrote:
> > > 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.
> > 
> > Hi Michal,
> > 
> > Yes, I've thought about it, and coccinelle falls a bit short (it's not smart
> > enough to know which put_page()'s to convert). However, there is a debug
> > option planned: a yet-to-be-posted commit [1] uses struct page extensions
> > (obviously protected by CONFIG_DEBUG_GET_USER_PAGES_REFERENCES) to add
> > a redundant counter. That allows:
> > 
> > void __put_page(struct page *page)
> > {
> >     ...
> >     /* Someone called put_page() instead of put_user_page() */
> >     WARN_ON_ONCE(atomic_read(&page_ext->pin_count) > 0);
> > 
> > > > > 
> > > > > 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?
> > 
> > The debug option above is still a bit simplistic in its implementation
> > (and maybe not taking full advantage of the data it has), but I think
> > it's preferable, because it monitors the "core" and WARNs.
> > 
> > Instead of the wrapper, I'm thinking: documentation and the passage of
> > time, plus the debug option (perhaps enhanced--probably once I post it
> > someone will notice opportunities), yes?
> 
> So I think your debug option and my suggested renaming serve a bit
> different purposes (and thus both make sense). If you do the renaming, you
> can just grep to see unconverted sites. Also when someone merges new GUP
> user (unaware of the new rules) while you switch GUP to use pins instead of
> ordinary references, you'll get compilation error in case of renaming
> instead of hard to debug refcount leak without the renaming. And such
> conflict is almost bound to happen given the size of GUP patch set... Also
> the renaming serves against the "coding inertia" - i.e., GUP is around for
> ages so people just use it without checking any documentation or comments.
> After switching how GUP works, what used to be correct isn't anymore so
> renaming the function serves as a warning that something has really
> changed.

Fully agreed!

> Your refcount debug patches are good to catch bugs in the conversions done
> but that requires you to be able to excercise the code path in the first
> place which may require particular HW or so, and you also have to enable
> the debug option which means you already aim at verifying the GUP
> references are treated properly.
> 
>                                                               Honza
> 
> -- 
> Jan Kara <jack@xxxxxxxx>
> SUSE Labs, CR

-- 
Michal Hocko
SUSE Labs

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