On Sat, 11 Nov 2017, Dan Williams wrote:
Currently only get_user_pages_fast() can safely handle the writable gup case due to its use of pud_access_permitted() to check whether the pud entry is writable. In the gup slow path pud_write() is used instead of pud_access_permitted() and to date it has been unimplemented, just calls BUG_ON().
kernel BUG at ./include/linux/hugetlb.h:244! [..] RIP: 0010:follow_devmap_pud+0x482/0x490 [..] Call Trace: follow_page_mask+0x28c/0x6e0 __get_user_pages+0xe4/0x6c0 get_user_pages_unlocked+0x130/0x1b0 get_user_pages_fast+0x89/0xb0 iov_iter_get_pages_alloc+0x114/0x4a0 nfs_direct_read_schedule_iovec+0xd2/0x350 ? nfs_start_io_direct+0x63/0x70 nfs_file_direct_read+0x1e0/0x250 nfs_file_read+0x90/0xc0For now this just implements a simple check for the _PAGE_RW bit similar to pmd_write. However, this implies that the gup-slow-path check is missing the extra checks that the gup-fast-path performs with pud_access_permitted. Later patches will align all checks to use the 'access_permitted' helper if the architecture provides it. Note that the generic 'access_permitted' helper fallback is the simple _PAGE_RW check on architectures that do not define the 'access_permitted' helper(s).
Fixes: a00cc7d9dd93 ("mm, x86: add support for PUD-sized transparent hugepages") Cc: Kirill A. Shutemov kirill.shutemov@linux.intel.com Cc: Catalin Marinas catalin.marinas@arm.com Cc: "David S. Miller" davem@davemloft.net Cc: Thomas Gleixner tglx@linutronix.de Cc: Dave Hansen dave.hansen@intel.com Cc: Will Deacon will.deacon@arm.com Cc: "H. Peter Anvin" hpa@zytor.com Cc: Ingo Molnar mingo@redhat.com Cc: Arnd Bergmann arnd@arndb.de Cc: stable@vger.kernel.org Cc: x86@kernel.org Signed-off-by: Dan Williams dan.j.williams@intel.com
arch/arm64/include/asm/pgtable.h | 1 + arch/sparc/include/asm/pgtable_64.h | 1 + arch/x86/include/asm/pgtable.h | 6 ++++++
For the x86 part:
Acked-by: Thomas Gleixner tglx@linutronix.de