@@ -1412,7 +1412,7 @@ void collapse_pte_mapped_thp(struct mm_s
{
unsigned long haddr = addr & HPAGE_PMD_MASK;
struct vm_area_struct *vma = find_vma(mm, haddr);
- struct page *hpage = NULL;
+ struct page *hpage;
pte_t *start_pte, *pte;
pmd_t *pmd, _pmd;
spinlock_t *ptl;
@@ -1432,9 +1432,17 @@ void collapse_pte_mapped_thp(struct mm_s
if (!hugepage_vma_check(vma, vma->vm_flags | VM_HUGEPAGE))
return;
+ hpage = find_lock_page(vma->vm_file->f_mapping,
+ linear_page_index(vma, haddr));
+ if (!hpage)
+ return;
+
+ if (!PageHead(hpage))
+ goto drop_hpage;
+
pmd = mm_find_pmd(mm, haddr);
if (!pmd)
- return;
+ goto drop_hpage;
start_pte = pte_offset_map_lock(mm, pmd, haddr, &ptl);
@@ -1453,30 +1461,11 @@ void collapse_pte_mapped_thp(struct mm_s
page = vm_normal_page(vma, addr, *pte);
- if (!page || !PageCompound(page))
- goto abort;
-
- if (!hpage) {
- hpage = compound_head(page);
- /*
- * The mapping of the THP should not change.
- *
- * Note that uprobe, debugger, or MAP_PRIVATE may
- * change the page table, but the new page will
- * not pass PageCompound() check.
- */
- if (WARN_ON(hpage->mapping != vma->vm_file->f_mapping))
- goto abort;
- }
-
/*
- * Confirm the page maps to the correct subpage.
- *
- * Note that uprobe, debugger, or MAP_PRIVATE may change
- * the page table, but the new page will not pass
- * PageCompound() check.
+ * Note that uprobe, debugger, or MAP_PRIVATE may change the
+ * page table, but the new page will not be a subpage of hpage.
*/
- if (WARN_ON(hpage + i != page))
+ if (hpage + i != page)
goto abort;
count++;
}
@@ -1495,7 +1484,7 @@ void collapse_pte_mapped_thp(struct mm_s
pte_unmap_unlock(start_pte, ptl);
/* step 3: set proper refcount and mm_counters. */
- if (hpage) {
+ if (count) {
page_ref_sub(hpage, count);
add_mm_counter(vma->vm_mm, mm_counter_file(hpage), -count);
}
@@ -1506,10 +1495,15 @@ void collapse_pte_mapped_thp(struct mm_s
spin_unlock(ptl);
mm_dec_nr_ptes(mm);
pte_free(mm, pmd_pgtable(_pmd));
+
+drop_hpage:
+ unlock_page(hpage);
+ put_page(hpage);
return;
abort:
pte_unmap_unlock(start_pte, ptl);
+ goto drop_hpage;
}
static int khugepaged_collapse_pte_mapped_thps(struct mm_slot *mm_slot)
When retract_page_tables() removes a page table to make way for a huge pmd, it holds huge page lock, i_mmap_lock_write, mmap_write_trylock and pmd lock; but when collapse_pte_mapped_thp() does the same (to handle the case when the original mmap_write_trylock had failed), only mmap_write_trylock and pmd lock are held. That's not enough. One machine has twice crashed under load, with "BUG: spinlock bad magic" and GPF on 6b6b6b6b6b6b6b6b. Examining the second crash, page_vma_mapped_walk_done()'s spin_unlock of pvmw->ptl (serving page_referenced() on a file THP, that had found a page table at *pmd) discovers that the page table page and its lock have already been freed by the time it comes to unlock. Follow the example of retract_page_tables(), but we only need one of huge page lock or i_mmap_lock_write to secure against this: because it's the narrower lock, and because it simplifies collapse_pte_mapped_thp() to know the hpage earlier, choose to rely on huge page lock here. Fixes: 27e1f8273113 ("khugepaged: enable collapse pmd for pte-mapped THP") Signed-off-by: Hugh Dickins <hughd@google.com> Cc: stable@vger.kernel.org # v5.4+ --- mm/khugepaged.c | 44 +++++++++++++++++++------------------------- 1 file changed, 19 insertions(+), 25 deletions(-)