Hi Michel, On Thu, Mar 24, 2011 at 10:00:16PM -0700, Michel Lespinasse wrote: > My question is this: After someone obtains a page reference using > get_user_pages(), what prevents them from getting additional > references with get_page() ? I always thought it was legal to > duplicate references that way, but now I don't see how it'd be safe > doing so on anon pages with THP enabled. It's not legal anymore as you noticed, but I'm not aware of anything doing that. I don't see an useful case where a driver could need to take one extra refcount after GUP returned. The normal API is GUP/put_page. We could make it legal again by taking the compound_lock after a PageCompound check though. I hope it's not needed though. It's unavoidable in put_page because put_page will run out of order with regard to __split_huge_page_refcount. But serializing get_page in GUP against __split_huge_page_refcount is automatic through the pmd_trans_splitting bit and needed for all page table walkers anyway. Maybe it's good idea to add a comment to transhuge.txt about that? I don't think I added it. Grepping for get_page in drivers doesn't show too many, they mostly run through the vm_ops->fault handler. Most important I can't see how possibly it could be useful to run a get_page after get_user_pages(FOLL_GET) returns. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>