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
Use pud_access_permitted() to implement pud_write(), a later cleanup can remove {pte,pmd,pud}_write and replace them with {pte,pmd,pud}_access_permitted() drectly so that we only have one set of helpers these kinds of checks. For now, implementing pud_write() simplifies -stable backports.
Cc: stable@vger.kernel.org Cc: Dave Hansen dave.hansen@intel.com Fixes: a00cc7d9dd93 ("mm, x86: add support for PUD-sized transparent hugepages") Signed-off-by: Dan Williams dan.j.williams@intel.com ---
Sending this as RFC for opinion on whether this should just be a pud_flags() & _PAGE_RW check, like pmd_write, or pud_access_permitted() that also takes protection keys into account.
include/linux/hugetlb.h | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-)
diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h index fbf5b31d47ee..6a142b240ef7 100644 --- a/include/linux/hugetlb.h +++ b/include/linux/hugetlb.h @@ -242,8 +242,7 @@ static inline int pgd_write(pgd_t pgd) #ifndef pud_write static inline int pud_write(pud_t pud) { - BUG(); - return 0; + return pud_access_permitted(pud, WRITE); } #endif