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charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org 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@gmail.com 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 > SUSE Labs, CR -- Michal Hocko SUSE Labs _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel