mm/khugepaged: write-lock VMA while collapsing a huge page

Protect VMA from concurrent page fault handler while collapsing a huge
page.  Page fault handler needs a stable PMD to use PTL and relies on
per-VMA lock to prevent concurrent PMD changes.  pmdp_collapse_flush(),
set_huge_pmd() and collapse_and_free_pmd() can modify a PMD, which will
not be detected by a page fault handler without proper locking.

Before this patch, page tables can be walked under any one of the
mmap_lock, the mapping lock, and the anon_vma lock; so when khugepaged
unlinks and frees page tables, it must ensure that all of those either are
locked or don't exist.  This patch adds a fourth lock under which page
tables can be traversed, and so khugepaged must also lock out that one.

[surenb@google.com: vm_lock/i_mmap_rwsem inversion in retract_page_tables]
  Link: https://lkml.kernel.org/r/20230303213250.3555716-1-surenb@google.com
[surenb@google.com: build fix]
  Link: https://lkml.kernel.org/r/CAJuCfpFjWhtzRE1X=J+_JjgJzNKhq-=JT8yTBSTHthwp0pqWZw@mail.gmail.com
Link: https://lkml.kernel.org/r/20230227173632.3292573-16-surenb@google.com
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

2 files changed, 24 insertions, 15 deletions

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index c7317678cb10..bee7fd7db380 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1056,6 +1056,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
 	if (result != SCAN_SUCCEED)
 		goto out_up_write;
 
+	vma_start_write(vma);
 	anon_vma_lock_write(vma->anon_vma);
 
 	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, address,
@@ -1517,6 +1518,9 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
 		goto drop_hpage;
 	}
 
+	/* Lock the vma before taking i_mmap and page table locks */
+	vma_start_write(vma);
+
 	/*
 	 * We need to lock the mapping so that from here on, only GUP-fast and
 	 * hardware page walks can access the parts of the page tables that
@@ -1694,6 +1698,10 @@ static int retract_page_tables(struct address_space *mapping, pgoff_t pgoff,
 		result = SCAN_PTE_MAPPED_HUGEPAGE;
 		if ((cc->is_khugepaged || is_target) &&
 		    mmap_write_trylock(mm)) {
+			/* trylock for the same lock inversion as above */
+			if (!vma_try_start_write(vma))
+				goto unlock_next;
+
 			/*
 			 * Re-check whether we have an ->anon_vma, because
 			 * collapse_and_free_pmd() requires that either no
diff --git a/mm/rmap.c b/mm/rmap.c
index 1ea2756b2797..ba901c416785 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -25,21 +25,22 @@
  *     mapping->invalidate_lock (in filemap_fault)
  *       page->flags PG_locked (lock_page)
  *         hugetlbfs_i_mmap_rwsem_key (in huge_pmd_share, see hugetlbfs below)
- *           mapping->i_mmap_rwsem
- *             anon_vma->rwsem
- *               mm->page_table_lock or pte_lock
- *                 swap_lock (in swap_duplicate, swap_info_get)
- *                   mmlist_lock (in mmput, drain_mmlist and others)
- *                   mapping->private_lock (in block_dirty_folio)
- *                     folio_lock_memcg move_lock (in block_dirty_folio)
- *                       i_pages lock (widely used)
- *                         lruvec->lru_lock (in folio_lruvec_lock_irq)
- *                   inode->i_lock (in set_page_dirty's __mark_inode_dirty)
- *                   bdi.wb->list_lock (in set_page_dirty's __mark_inode_dirty)
- *                     sb_lock (within inode_lock in fs/fs-writeback.c)
- *                     i_pages lock (widely used, in set_page_dirty,
- *                               in arch-dependent flush_dcache_mmap_lock,
- *                               within bdi.wb->list_lock in __sync_single_inode)
+ *           vma_start_write
+ *             mapping->i_mmap_rwsem
+ *               anon_vma->rwsem
+ *                 mm->page_table_lock or pte_lock
+ *                   swap_lock (in swap_duplicate, swap_info_get)
+ *                     mmlist_lock (in mmput, drain_mmlist and others)
+ *                     mapping->private_lock (in block_dirty_folio)
+ *                       folio_lock_memcg move_lock (in block_dirty_folio)
+ *                         i_pages lock (widely used)
+ *                           lruvec->lru_lock (in folio_lruvec_lock_irq)
+ *                     inode->i_lock (in set_page_dirty's __mark_inode_dirty)
+ *                     bdi.wb->list_lock (in set_page_dirty's __mark_inode_dirty)
+ *                       sb_lock (within inode_lock in fs/fs-writeback.c)
+ *                       i_pages lock (widely used, in set_page_dirty,
+ *                                 in arch-dependent flush_dcache_mmap_lock,
+ *                                 within bdi.wb->list_lock in __sync_single_inode)
  *
  * anon_vma->rwsem,mapping->i_mmap_rwsem   (memory_failure, collect_procs_anon)
  *   ->tasklist_lock