[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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

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 
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?

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.

Yes, there are quite a few places that don't involve _bvec, as we can see
right here. So we need something. Andrew asked for a debug option some time
ago, and several people (Dave Hansen, Dan Williams, Jerome) had the idea
of vmap-ing gup pages separately, so you can definitely tell where each
page came from. I'm hoping not to have to go to that level of complexity

[1] "mm/gup: debug tracking of get_user_pages() references" :

John Hubbard

Xen-devel mailing list



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.