[PATCH] mm: fix double page fault on arm64 if PTE_AF is cleared

Jia He
2019-09-04T00:58:48Z

When we tested pmdk unit test [1] vmmalloc_fork TEST1 in arm64 guest, there
will be a double page fault in __copy_from_user_inatomic of cow_user_page.

Below call trace is from arm64 do_page_fault for debugging purpose
[ 110.016195] Call trace:
[ 110.016826] do_page_fault+0x5a4/0x690
[ 110.017812] do_mem_abort+0x50/0xb0
[ 110.018726] el1_da+0x20/0xc4
[ 110.019492] __arch_copy_from_user+0x180/0x280
[ 110.020646] do_wp_page+0xb0/0x860
[ 110.021517] __handle_mm_fault+0x994/0x1338
[ 110.022606] handle_mm_fault+0xe8/0x180
[ 110.023584] do_page_fault+0x240/0x690
[ 110.024535] do_mem_abort+0x50/0xb0
[ 110.025423] el0_da+0x20/0x24

The pte info before __copy_from_user_inatomic is(PTE_AF is cleared):
[ffff9b007000] pgd=000000023d4f8003, pud=000000023da9b003, pmd=000000023d4b3003, pte=360000298607bd3

The keypoint is: we don't always have a hardware-managed access flag on
arm64.

The root cause is in copy_one_pte, it will clear the PTE_AF for COW
pages. Generally, when it is accessed by user, the COW pages will be set
as accessed(PTE_AF bit on arm64) by hardware if hardware feature is
supported. But on some arm64 platforms, the PTE_AF needs to be set by
software.

This patch fix it by calling pte_mkyoung. Also, the parameter is
changed because vmf should be passed to cow_user_page()

[1] https://github.com/pmem/pmdk/tree/master/src/test/vmmalloc_fork

Reported-by: Yibo Cai <Yibo.Cai@arm.com>
Signed-off-by: Jia He <justin.he@arm.com>
---
mm/memory.c | 21 ++++++++++++++++-----
1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index e2bb51b6242e..b1f9ace2e943 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2140,7 +2140,8 @@ static inline int pte_unmap_same(struct mm_struct *mm, pmd_t *pmd,
return same;
}

-static inline void cow_user_page(struct page *dst, struct page *src, unsigned long va, struct vm_area_struct *vma)
+static inline void cow_user_page(struct page *dst, struct page *src,
+ struct vm_fault *vmf)
{
debug_dma_assert_idle(src);

@@ -2152,20 +2153,30 @@ static inline void cow_user_page(struct page *dst, struct page *src, unsigned lo
*/
if (unlikely(!src)) {
void *kaddr = kmap_atomic(dst);
- void __user *uaddr = (void __user *)(va & PAGE_MASK);
+ void __user *uaddr = (void __user *)(vmf->address & PAGE_MASK);
+ pte_t entry;

/*
* This really shouldn't fail, because the page is there
* in the page tables. But it might just be unreadable,
* in which case we just give up and fill the result with
- * zeroes.
+ * zeroes. If PTE_AF is cleared on arm64, it might
+ * cause double page fault here. so makes pte young here
*/
+ if (!pte_young(vmf->orig_pte)) {
+ entry = pte_mkyoung(vmf->orig_pte);
+ if (ptep_set_access_flags(vmf->vma, vmf->address,
+ vmf->pte, entry, vmf->flags & FAULT_FLAG_WRITE))
+ update_mmu_cache(vmf->vma, vmf->address,
+ vmf->pte);
+ }
+
if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE))
clear_page(kaddr);
kunmap_atomic(kaddr);
flush_dcache_page(dst);
} else
- copy_user_highpage(dst, src, va, vma);
+ copy_user_highpage(dst, src, vmf->address, vmf->vma);
}

static gfp_t __get_fault_gfp_mask(struct vm_area_struct *vma)
@@ -2318,7 +2329,7 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf)
vmf->address);
if (!new_page)
goto oom;
- cow_user_page(new_page, old_page, vmf->address, vma);
+ cow_user_page(new_page, old_page, vmf);
}

if (mem_cgroup_try_charge_delay(new_page, mm, GFP_KERNEL, &memcg, false))
--
2.17.1