[PATCH v2 01/10] mm/thp: fix __split_huge_pmd_locked() on shmem migration entry (fwd)

[Hugh Dickins]

---------- Forwarded message ----------
Date: Tue, 8 Jun 2021 21:00:12 -0700 (PDT)
From: Hugh Dickins <hughd@xxxxxxxxxx>
To: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>,
    Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>,
    Yang Shi <shy828301@xxxxxxxxx>, Wang Yugui <wangyugui@xxxxxxxxxxxx>,
    Matthew Wilcox <willy@xxxxxxxxxxxxx>,
    Naoya Horiguchi <naoya.horiguchi@xxxxxxx>,
    Alistair Popple <apopple@xxxxxxxxxx>, Ralph Campbell <rcampbell@xxxxxxxxxx>,
    Zi Yan <ziy@xxxxxxxxxx>, Miaohe Lin <linmiaohe@xxxxxxxxxx>,
    Minchan Kim <minchan@xxxxxxxxxx>, Jue Wang <juew@xxxxxxxxxx>,
    Peter Xu <peterx@xxxxxxxxxx>, Jan Kara <jack@xxxxxxx>,
    Shakeel Butt <shakeelb@xxxxxxxxxx>, Oscar Salvador <osalvador@xxxxxxx>
Subject: [PATCH v2 01/10] mm/thp: fix __split_huge_pmd_locked() on shmem
    migration entry

Stressing huge tmpfs page migration racing hole punch often crashed on the
VM_BUG_ON(!pmd_present) in pmdp_huge_clear_flush(), with DEBUG_VM=y kernel;
or shortly afterwards, on a bad dereference in __split_huge_pmd_locked()
when DEBUG_VM=n.  They forgot to allow for pmd migration entries in the
non-anonymous case.

Full disclosure: those particular experiments were on a kernel with more
relaxed mmap_lock and i_mmap_rwsem locking, and were not repeated on the
vanilla kernel: it is conceivable that stricter locking happens to avoid
those cases, or makes them less likely; but __split_huge_pmd_locked()
already allowed for pmd migration entries when handling anonymous THPs,
so this commit brings the shmem and file THP handling into line.

And while there: use old_pmd rather than _pmd, as in the following blocks;
and make it clearer to the eye that the !vma_is_anonymous() block is
self-contained, making an early return after accounting for unmapping.

Fixes: e71769ae5260 ("mm: enable thp migration for shmem thp")
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
---
v2: omit is_huge_zero_pmd() mods (done differently in next), per Kirill

 mm/huge_memory.c     | 27 ++++++++++++++++++---------
 mm/pgtable-generic.c |  5 ++---
 2 files changed, 20 insertions(+), 12 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 63ed6b25deaa..42cfefc6e66e 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2044,7 +2044,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 	count_vm_event(THP_SPLIT_PMD);
 
 	if (!vma_is_anonymous(vma)) {
-		_pmd = pmdp_huge_clear_flush_notify(vma, haddr, pmd);
+		old_pmd = pmdp_huge_clear_flush_notify(vma, haddr, pmd);
 		/*
 		 * We are going to unmap this huge page. So
 		 * just go ahead and zap it
@@ -2053,16 +2053,25 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 			zap_deposited_table(mm, pmd);
 		if (vma_is_special_huge(vma))
 			return;
-		page = pmd_page(_pmd);
-		if (!PageDirty(page) && pmd_dirty(_pmd))
-			set_page_dirty(page);
-		if (!PageReferenced(page) && pmd_young(_pmd))
-			SetPageReferenced(page);
-		page_remove_rmap(page, true);
-		put_page(page);
+		if (unlikely(is_pmd_migration_entry(old_pmd))) {
+			swp_entry_t entry;
+
+			entry = pmd_to_swp_entry(old_pmd);
+			page = migration_entry_to_page(entry);
+		} else {
+			page = pmd_page(old_pmd);
+			if (!PageDirty(page) && pmd_dirty(old_pmd))
+				set_page_dirty(page);
+			if (!PageReferenced(page) && pmd_young(old_pmd))
+				SetPageReferenced(page);
+			page_remove_rmap(page, true);
+			put_page(page);
+		}
 		add_mm_counter(mm, mm_counter_file(page), -HPAGE_PMD_NR);
 		return;
-	} else if (pmd_trans_huge(*pmd) && is_huge_zero_pmd(*pmd)) {
+	}
+
+	if (pmd_trans_huge(*pmd) && is_huge_zero_pmd(*pmd)) {
 		/*
 		 * FIXME: Do we want to invalidate secondary mmu by calling
 		 * mmu_notifier_invalidate_range() see comments below inside
diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
index c2210e1cdb51..4e640baf9794 100644
--- a/mm/pgtable-generic.c
+++ b/mm/pgtable-generic.c
@@ -135,9 +135,8 @@ pmd_t pmdp_huge_clear_flush(struct vm_area_struct *vma, unsigned long address,
 {
 	pmd_t pmd;
 	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
-	VM_BUG_ON(!pmd_present(*pmdp));
-	/* Below assumes pmd_present() is true */
-	VM_BUG_ON(!pmd_trans_huge(*pmdp) && !pmd_devmap(*pmdp));
+	VM_BUG_ON(pmd_present(*pmdp) && !pmd_trans_huge(*pmdp) &&
+			   !pmd_devmap(*pmdp));
 	pmd = pmdp_huge_get_and_clear(vma->vm_mm, address, pmdp);
 	flush_pmd_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
 	return pmd;
-- 
2.26.2