mm: fix kernel BUG at huge_memory.c:1474!

Andrea's autonuma-benchmark numa01 hits kernel BUG at huge_memory.c:1474!
in change_huge_pmd called from change_protection from change_prot_numa
from task_numa_work.

That BUG, introduced in the huge zero page commit cad7f613c4d0 ("thp:
change_huge_pmd(): make sure we don't try to make a page writable")
was trying to verify that newprot never adds write permission to an
anonymous huge page; but Automatic NUMA Balancing's 4b10e7d562c9 ("mm:
mempolicy: Implement change_prot_numa() in terms of change_protection()")
adds a new prot_numa path into change_huge_pmd(), which makes no use of
the newprot provided, and may retain the write bit in the pmd.

Just move the BUG_ON(pmd_write(entry)) up into the !prot_numa block.

Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 3 deletions

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index d7ee1691fd21..32754eece63e 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1460,9 +1460,10 @@ int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
         if (__pmd_trans_huge_lock(pmd, vma) == 1) {
                 pmd_t entry;
                 entry = pmdp_get_and_clear(mm, addr, pmd);
-                if (!prot_numa)
+                if (!prot_numa) {
                         entry = pmd_modify(entry, newprot);
-                else {
+                        BUG_ON(pmd_write(entry));
+                } else {
                         struct page *page = pmd_page(*pmd);
 
                         /* only check non-shared pages */
@@ -1471,7 +1472,6 @@ int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
                                 entry = pmd_mknuma(entry);
                         }
                 }
-                BUG_ON(pmd_write(entry));
                 set_pmd_at(mm, addr, pmd, entry);
                 spin_unlock(&vma->vm_mm->page_table_lock);
                 ret = 1;