From: Dan Williams dan.j.williams@intel.com Subject: mm: fix device-dax pud write-faults triggered by get_user_pages()

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+0x90/0xc0

For 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).

[dan.j.williams@intel.com: fix powerpc compile error] Link: http://lkml.kernel.org/r/151129126165.37405.16031785266675461397.stgit@dwill... Link: http://lkml.kernel.org/r/151043109938.2842.14834662818213616199.stgit@dwilli... Fixes: a00cc7d9dd93 ("mm, x86: add support for PUD-sized transparent hugepages") Signed-off-by: Dan Williams dan.j.williams@intel.com Reported-by: Stephen Rothwell sfr@canb.auug.org.au Acked-by: Thomas Gleixner tglx@linutronix.de [x86] Cc: Kirill A. Shutemov kirill.shutemov@linux.intel.com Cc: Catalin Marinas catalin.marinas@arm.com Cc: "David S. Miller" davem@davemloft.net 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 Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

arch/x86/include/asm/pgtable.h | 6 ++++++ include/asm-generic/pgtable.h | 8 ++++++++ include/linux/hugetlb.h | 8 -------- 3 files changed, 14 insertions(+), 8 deletions(-)

diff -puN arch/x86/include/asm/pgtable.h~mm-fix-device-dax-pud-write-faults-triggered-by-get_user_pages arch/x86/include/asm/pgtable.h --- a/arch/x86/include/asm/pgtable.h~mm-fix-device-dax-pud-write-faults-triggered-by-get_user_pages +++ a/arch/x86/include/asm/pgtable.h @@ -1088,6 +1088,12 @@ static inline void pmdp_set_wrprotect(st clear_bit(_PAGE_BIT_RW, (unsigned long *)pmdp); }

+#define pud_write pud_write +static inline int pud_write(pud_t pud) +{ + return pud_flags(pud) & _PAGE_RW; +} + /* * clone_pgd_range(pgd_t *dst, pgd_t *src, int count); * diff -puN include/asm-generic/pgtable.h~mm-fix-device-dax-pud-write-faults-triggered-by-get_user_pages include/asm-generic/pgtable.h --- a/include/asm-generic/pgtable.h~mm-fix-device-dax-pud-write-faults-triggered-by-get_user_pages +++ a/include/asm-generic/pgtable.h @@ -814,6 +814,14 @@ static inline int pmd_write(pmd_t pmd) #endif /* __HAVE_ARCH_PMD_WRITE */ #endif /* CONFIG_TRANSPARENT_HUGEPAGE */

+#ifndef pud_write +static inline int pud_write(pud_t pud) +{ + BUG(); + return 0; +} +#endif /* pud_write */ + #if !defined(CONFIG_TRANSPARENT_HUGEPAGE) || \ (defined(CONFIG_TRANSPARENT_HUGEPAGE) && \ !defined(CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD)) diff -puN include/linux/hugetlb.h~mm-fix-device-dax-pud-write-faults-triggered-by-get_user_pages include/linux/hugetlb.h --- a/include/linux/hugetlb.h~mm-fix-device-dax-pud-write-faults-triggered-by-get_user_pages +++ a/include/linux/hugetlb.h @@ -239,14 +239,6 @@ static inline int pgd_write(pgd_t pgd) } #endif

-#ifndef pud_write -static inline int pud_write(pud_t pud) -{ - BUG(); - return 0; -} -#endif - #define HUGETLB_ANON_FILE "anon_hugepage"

enum { _