linux-kernel - [PATCH mm-unstable] mm: folio_add_new_anon_rmap() careful __folio_set

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Date: Mon, 24 Jun 2024 22:00:24 -0700 (PDT)
From: Hugh Dickins <hughd@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
cc: Barry Song <21cnbao@...il.com>, Hugh Dickins <hughd@...gle.com>, 
    baolin.wang@...ux.alibaba.com, chrisl@...nel.org, david@...hat.com, 
    linux-kernel@...r.kernel.org, linux-mm@...ck.org, mhocko@...e.com, 
    ryan.roberts@....com, shy828301@...il.com, surenb@...gle.com, 
    v-songbaohua@...o.com, willy@...radead.org, ying.huang@...el.com, 
    yosryahmed@...gle.com, yuanshuai@...o.com, yuzhao@...gle.com
Subject: [PATCH mm-unstable] mm: folio_add_new_anon_rmap() careful
 __folio_set_swapbacked()

Commit "mm: use folio_add_new_anon_rmap() if folio_test_anon(folio)==
false" has extended folio_add_new_anon_rmap() to use on non-exclusive
folios, already visible to others in swap cache and on LRU.

That renders its non-atomic __folio_set_swapbacked() unsafe: it risks
overwriting concurrent atomic operations on folio->flags, losing bits
added or restoring bits cleared.  Since it's only used in this risky
way when folio_test_locked and !folio_test_anon, many such races are
excluded; but, for example, isolations by folio_test_clear_lru() are
vulnerable, and setting or clearing active.

It could just use the atomic folio_set_swapbacked(); but this function
does try to avoid atomics where it can, so use a branch instead: just
avoid setting swapbacked when it is already set, that is good enough.
(Swapbacked is normally stable once set: lazyfree can undo it, but
only later, when found anon in a page table.)

This fixes a lot of instability under compaction and swapping loads:
assorted "Bad page"s, VM_BUG_ON_FOLIO()s, apparently even page double
frees - though I've not worked out what races could lead to the latter.

Signed-off-by: Hugh Dickins <hughd@...gle.com>
---
 mm/rmap.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index df1a43295c85..5394c1178bf1 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1408,7 +1408,9 @@ void folio_add_new_anon_rmap(struct folio *folio, struct vm_area_struct *vma,
 	VM_WARN_ON_FOLIO(folio_test_hugetlb(folio), folio);
 	VM_BUG_ON_VMA(address < vma->vm_start ||
 			address + (nr << PAGE_SHIFT) > vma->vm_end, vma);
-	__folio_set_swapbacked(folio);
+
+	if (!folio_test_swapbacked(folio))
+		__folio_set_swapbacked(folio);
 	__folio_set_anon(folio, vma, address, exclusive);

 	if (likely(!folio_test_large(folio))) {
-- 
2.35.3