Patch "arm64: hugetlb: remove the wrong pmd check in find_num_contig()" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

arm64: hugetlb: remove the wrong pmd check in find_num_contig()

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
arm64-hugetlb-remove-the-wrong-pmd-check-in-find_num_contig.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Huang Shijie <[email protected]>
Date: Tue, 8 Nov 2016 13:44:38 +0800
Subject: arm64: hugetlb: remove the wrong pmd check in find_num_contig()

From: Huang Shijie <[email protected]>

commit 20156ce2365d61beaa6f5a78a7a789044e0e7acc upstream.

The find_num_contig() will return 1 when the pmd is not present. It will cause a kernel dead loop in the following scenaro:

1.) pmd entry is not present.

2.) the page fault occurs:

       ... hugetlb_fault() --> hugetlb_no_page() --> set_huge_pte_at()

3.) set_huge_pte_at() will only set the first PMD entry, since the
find_num_contig just return 1 in this case. So the PMD entries are all empty except the first one.

4.) when kernel accesses the address mapped by the second PMD entry,
a new page fault occurs:
... hugetlb_fault() --> huge_ptep_set_access_flags()

The second PMD entry is still empty now.

5.) When the kernel returns, the access will cause a page fault again.
The kernel will run like the "4)" above.
We will see a dead loop since here.

The dead loop is caught in the 32M hugetlb page (2M PMD + Contiguous bit).

This patch removes wrong pmd check, and fixes this dead loop.

This patch also removes the redundant checks for PGD/PUD in the find_num_contig().

---
 arch/arm64/mm/hugetlbpage.c |   12 ------------
 1 file changed, 12 deletions(-)

@@ -51,20 +51,8 @@ static int find_num_contig(struct mm_str
 	*pgsize = PAGE_SIZE;
 	if (!pte_cont(pte))
 		return 1;
-	if (!pgd_present(*pgd)) {
-		VM_BUG_ON(!pgd_present(*pgd));
-		return 1;
-	}
 	pud = pud_offset(pgd, addr);
-	if (!pud_present(*pud)) {
-		VM_BUG_ON(!pud_present(*pud));
-		return 1;
-	}
 	pmd = pmd_offset(pud, addr);
-	if (!pmd_present(*pmd)) {
-		VM_BUG_ON(!pmd_present(*pmd));
-		return 1;
-	}
 	if ((pte_t *)pmd == ptep) {
 		*pgsize = PMD_SIZE;
 		return CONT_PMDS;

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/arm64-hugetlb-fix-the-wrong-address-for-several-functions.patch queue-4.9/arm64-hugetlb-remove-the-wrong-pmd-check-in-find_num_contig.patch queue-4.9/arm64-hugetlb-fix-the-wrong-return-value-for-huge_ptep_set_access_flags.patch