On Wed, Dec 2, 2015 at 2:00 PM, Dave Hansen <dave.hansen@xxxxxxxxx> wrote: > On 12/02/2015 12:54 PM, Dan Williams wrote: >> On Wed, Dec 2, 2015 at 1:37 PM, Toshi Kani <toshi.kani@xxxxxxx> wrote: >>> > On Wed, 2015-12-02 at 11:57 -0800, Dan Williams wrote: >> [..] >>>> >> The whole point of __get_user_page_fast() is to avoid the overhead of >>>> >> taking the mm semaphore to access the vma. _PAGE_SPECIAL simply tells >>>> >> __get_user_pages_fast that it needs to fallback to the >>>> >> __get_user_pages slow path. >>> > >>> > I see. Then, I think gup_huge_pmd() can simply return 0 when !pfn_valid(), >>> > instead of VM_BUG_ON. >> Is pfn_valid() a reliable check? It seems to be based on a max_pfn >> per node... what happens when pmem is located below that point. I >> haven't been able to convince myself that we won't get false >> positives, but maybe I'm missing something. > > With sparsemem at least, it makes sure that you're looking at a valid > _section_. See the pfn_valid() at ~include/linux/mmzone.h:1222. At a minimum we would need to add "depends on SPARSEMEM" to "config FS_DAX_PMD". -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>