On Tue, Sep 24, 2019 at 06:43:06AM +0000, Justin He (Arm Technology China) wrote:

Catalin Marinas wrote:

quoted

On Sat, Sep 21, 2019 at 09:50:54PM +0800, Jia He wrote:

quoted

@@ -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.
+		 */

[...]

quoted

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+		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 false;
+			}
+			pte_unmap_unlock(vmf->pte, vmf->ptl);
+		}

[...]

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quoted

+
+		kaddr = kmap_atomic(dst);

Since you moved the kmap_atomic() here, could the above
arch_faults_on_old_pte() run in a preemptible context? I suggested to
add a WARN_ON in patch 2 to be sure.

Should I move kmap_atomic back to the original line? Thus, we can make sure
that arch_faults_on_old_pte() is in the context of preempt_disabled?
Otherwise, arch_faults_on_old_pte() may cause plenty of warning if I add
a WARN_ON in arch_faults_on_old_pte.  I tested it when I enable the PREEMPT=y
on a ThunderX2 qemu guest.

So we have two options here:

1. Change arch_faults_on_old_pte() scope to the whole system rather than
   just the current CPU. You'd have to wire up a new arm64 capability
   for the access flag but this way we don't care whether it's
   preemptible or not.

2. Keep the arch_faults_on_old_pte() per-CPU but make sure we are not
   preempted here. The kmap_atomic() move would do but you'd have to
   kunmap_atomic() before the return.

I think the answer to my question below also has some implication on
which option to pick:

quoted

quoted

 		/*
 		 * 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);

That's more of a question for the mm guys: at this point we do the
copying with the ptl released; is there anything else that could have
made the pte old in the meantime? I think unuse_pte() is only called on
anonymous vmas, so it shouldn't be the case here.

If we need to hold the ptl here, you could as well have an enclosing
kmap/kunmap_atomic (option 2) with some goto instead of "return false".

-- 
Catalin

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