On Aug 1, 2019, at 5:43 AM, Oleg Nesterov [off-list ref] wrote:

On 07/31, Song Liu wrote:

quoted

+void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long haddr)
+{
+	struct vm_area_struct *vma = find_vma(mm, haddr);
+	pmd_t *pmd = mm_find_pmd(mm, haddr);
+	struct page *hpage = NULL;
+	unsigned long addr;
+	spinlock_t *ptl;
+	int count = 0;
+	pmd_t _pmd;
+	int i;
+
+	VM_BUG_ON(haddr & ~HPAGE_PMD_MASK);
+
+	if (!vma || !pmd || pmd_trans_huge(*pmd))

                           ^^^^^^^^^^^^^^^^^^^^

mm_find_pmd() returns NULL if pmd_trans_huge()

Good catch! I will simplify this one in v3. 

quoted

+	/* step 1: check all mapped PTEs are to the right huge page */
+	for (i = 0, addr = haddr; i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE) {
+		pte_t *pte = pte_offset_map(pmd, addr);
+		struct page *page;
+
+		if (pte_none(*pte))
+			continue;
+
+		page = vm_normal_page(vma, addr, *pte);
+
+		if (!PageCompound(page))
+			return;
+
+		if (!hpage) {
+			hpage = compound_head(page);
+			if (hpage->mapping != vma->vm_file->f_mapping)

Hmm. But how can we know this is still the same vma ?

If nothing else, why vma->vm_file can't be NULL?

Good point. We should confirm vma->vm_file is not NULL. 

Thanks,
Song