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
As told by Catalin: "On arm64 without hardware Access Flag, copying from
user will fail because the pte is old and cannot be marked young. So we
always end up with zeroed page after fork() + CoW for pfn mappings. we
don't always have a hardware-managed access flag on arm64."
This patch fix it by calling pte_mkyoung. Also, the parameter is
changed because vmf should be passed to cow_user_page()
Add a WARN_ON_ONCE when __copy_from_user_inatomic() returns error
in case there can be some obscure use-case.(by Kirill)
[1] https://github.com/pmem/pmdk/tree/master/src/test/vmmalloc_fork
Reported-by: Yibo Cai <[email protected]>
Signed-off-by: Jia He <[email protected]>
---
mm/memory.c | 67 ++++++++++++++++++++++++++++++++++++++++++++++++-----
1 file changed, 61 insertions(+), 6 deletions(-)
diff --git a/mm/memory.c b/mm/memory.c
index e2bb51b6242e..3e39e40fee87 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -118,6 +118,13 @@ int randomize_va_space __read_mostly =
2;
#endif
+#ifndef arch_faults_on_old_pte
+static inline bool arch_faults_on_old_pte(void)
+{
+ return false;
+}
+#endif
+
static int __init disable_randmaps(char *s)
{
randomize_va_space = 0;
@@ -2140,8 +2147,13 @@ 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 int cow_user_page(struct page *dst, struct page *src,
+ struct vm_fault *vmf)
{
+ struct vm_area_struct *vma = vmf->vma;
+ struct mm_struct *mm = vma->vm_mm;
+ unsigned long addr = vmf->address;
+
debug_dma_assert_idle(src);
/*
@@ -2151,21 +2163,53 @@ static inline void cow_user_page(struct page *dst, struct page *src, unsigned lo
* fails, we just zero-fill it. Live with it.
*/
if (unlikely(!src)) {
- void *kaddr = kmap_atomic(dst);
- void __user *uaddr = (void __user *)(va & PAGE_MASK);
+ void *kaddr;
+ pte_t entry;
+ void __user *uaddr = (void __user *)(addr & PAGE_MASK);
+ /* On architectures with software "accessed" bits, we would
+ * take a double page fault, so mark it accessed here.
+ */
+ if (arch_faults_on_old_pte() && !pte_young(vmf->orig_pte)) {
+ vmf->pte = pte_offset_map_lock(mm, vmf->pmd, addr,
+ &vmf->ptl);
+ if (likely(pte_same(*vmf->pte, vmf->orig_pte))) {
+ entry = pte_mkyoung(vmf->orig_pte);
+ if (ptep_set_access_flags(vma, addr,
+ vmf->pte, entry, 0))
+ update_mmu_cache(vma, addr, vmf->pte);
+ } else {
+ /* Other thread has already handled the fault
+ * and we don't need to do anything. If it's
+ * not the case, the fault will be triggered
+ * again on the same address.
+ */
+ pte_unmap_unlock(vmf->pte, vmf->ptl);
+ return -1;
+ }
+ pte_unmap_unlock(vmf->pte, vmf->ptl);
+ }
+
+ kaddr = kmap_atomic(dst);
/*
* 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.
*/
- if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE))
+ if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE)) {
+ /* Give a warn in case there can be some obscure
+ * use-case
+ */
+ WARN_ON_ONCE(1);
clear_page(kaddr);
+ }
kunmap_atomic(kaddr);
flush_dcache_page(dst);
} else
- copy_user_highpage(dst, src, va, vma);
+ copy_user_highpage(dst, src, addr, vma);
+
+ return 0;
}
static gfp_t __get_fault_gfp_mask(struct vm_area_struct *vma)
@@ -2318,7 +2362,18 @@ 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);
+
+ if (cow_user_page(new_page, old_page, vmf)) {
+ /* COW failed, if the fault was solved by other,
+ * it's fine. If not, userspace would re-fault on
+ * the same address and we will handle the fault
+ * from the second attempt.
+ */
+ put_page(new_page);
+ if (old_page)
+ put_page(old_page);
+ return 0;
+ }
}
if (mem_cgroup_try_charge_delay(new_page, mm, GFP_KERNEL, &memcg, false))
--
2.17.1
[On behalf of [email protected]]
Hi Matthew
On 2019/9/20 23:53, Matthew Wilcox wrote:
> On Fri, Sep 20, 2019 at 09:54:37PM +0800, Jia He wrote:
>> -static inline void cow_user_page(struct page *dst, struct page *src, unsigned long va, struct vm_area_struct *vma)
>> +static inline int cow_user_page(struct page *dst, struct page *src,
>> + struct vm_fault *vmf)
>> {
> Can we talk about the return type here?
>
>> + } else {
>> + /* Other thread has already handled the fault
>> + * and we don't need to do anything. If it's
>> + * not the case, the fault will be triggered
>> + * again on the same address.
>> + */
>> + pte_unmap_unlock(vmf->pte, vmf->ptl);
>> + return -1;
> ...
>> + return 0;
>> }
> So -1 for "try again" and 0 for "succeeded".
>
>> + if (cow_user_page(new_page, old_page, vmf)) {
> Then we use it like a bool. But it's kind of backwards from a bool because
> false is success.
>
>> + /* COW failed, if the fault was solved by other,
>> + * it's fine. If not, userspace would re-fault on
>> + * the same address and we will handle the fault
>> + * from the second attempt.
>> + */
>> + put_page(new_page);
>> + if (old_page)
>> + put_page(old_page);
>> + return 0;
> And we don't use the return value; in fact we invert it.
>
> Would this make more sense:
>
> static inline bool cow_user_page(struct page *dst, struct page *src,
> struct vm_fault *vmf)
> ...
> pte_unmap_unlock(vmf->pte, vmf->ptl);
> return false;
> ...
> return true;
> ...
> if (!cow_user_page(new_page, old_page, vmf)) {
>
> That reads more sensibly for me. We could also go with returning a
> vm_fault_t, but that would be more complex than needed today, I think.
Ok, will change the return type to bool as you suggested.
Thanks
---
Cheers,
Justin (Jia He)
On Fri, Sep 20, 2019 at 09:54:37PM +0800, Jia He wrote:
> -static inline void cow_user_page(struct page *dst, struct page *src, unsigned long va, struct vm_area_struct *vma)
> +static inline int cow_user_page(struct page *dst, struct page *src,
> + struct vm_fault *vmf)
> {
Can we talk about the return type here?
> + } else {
> + /* Other thread has already handled the fault
> + * and we don't need to do anything. If it's
> + * not the case, the fault will be triggered
> + * again on the same address.
> + */
> + pte_unmap_unlock(vmf->pte, vmf->ptl);
> + return -1;
...
> + return 0;
> }
So -1 for "try again" and 0 for "succeeded".
> + if (cow_user_page(new_page, old_page, vmf)) {
Then we use it like a bool. But it's kind of backwards from a bool because
false is success.
> + /* COW failed, if the fault was solved by other,
> + * it's fine. If not, userspace would re-fault on
> + * the same address and we will handle the fault
> + * from the second attempt.
> + */
> + put_page(new_page);
> + if (old_page)
> + put_page(old_page);
> + return 0;
And we don't use the return value; in fact we invert it.
Would this make more sense:
static inline bool cow_user_page(struct page *dst, struct page *src,
struct vm_fault *vmf)
...
pte_unmap_unlock(vmf->pte, vmf->ptl);
return false;
...
return true;
...
if (!cow_user_page(new_page, old_page, vmf)) {
That reads more sensibly for me. We could also go with returning a
vm_fault_t, but that would be more complex than needed today, I think.
On Fri, Sep 20, 2019 at 08:53:00AM -0700, Matthew Wilcox wrote:
> On Fri, Sep 20, 2019 at 09:54:37PM +0800, Jia He wrote:
> > -static inline void cow_user_page(struct page *dst, struct page *src, unsigned long va, struct vm_area_struct *vma)
> > +static inline int cow_user_page(struct page *dst, struct page *src,
> > + struct vm_fault *vmf)
> > {
>
> Can we talk about the return type here?
>
> > + } else {
> > + /* Other thread has already handled the fault
> > + * and we don't need to do anything. If it's
> > + * not the case, the fault will be triggered
> > + * again on the same address.
> > + */
> > + pte_unmap_unlock(vmf->pte, vmf->ptl);
> > + return -1;
> ...
> > + return 0;
> > }
>
> So -1 for "try again" and 0 for "succeeded".
>
> > + if (cow_user_page(new_page, old_page, vmf)) {
>
> Then we use it like a bool. But it's kind of backwards from a bool because
> false is success.
>
> > + /* COW failed, if the fault was solved by other,
> > + * it's fine. If not, userspace would re-fault on
> > + * the same address and we will handle the fault
> > + * from the second attempt.
> > + */
> > + put_page(new_page);
> > + if (old_page)
> > + put_page(old_page);
> > + return 0;
>
> And we don't use the return value; in fact we invert it.
>
> Would this make more sense:
>
> static inline bool cow_user_page(struct page *dst, struct page *src,
> struct vm_fault *vmf)
> ...
> pte_unmap_unlock(vmf->pte, vmf->ptl);
> return false;
> ...
> return true;
> ...
> if (!cow_user_page(new_page, old_page, vmf)) {
>
> That reads more sensibly for me.
I like this idea too.
--
Kirill A. Shutemov