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Date:   Wed, 28 Jun 2023 21:31:35 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
To:     Andrew Morton <akpm@linux-foundation.org>
cc:     Matthew Wilcox <willy@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        David Stevens <stevensd@chromium.org>,
        Peter Xu <peterx@redhat.com>,
        "Kirill A . Shutemov" <kirill@shutemov.name>,
        Yang Shi <shy828301@gmail.com>,
        David Hildenbrand <david@redhat.com>,
        Jiaqi Yan <jiaqiyan@google.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: [PATCH hotfix] mm/khugepaged: fix regression in collapse_file()
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References: <20230607053135.2087354-1-stevensd@google.com> <f18e4b64-3f88-a8ab-56cc-d1f5f9c58d4@google.com> <CAD=HUj4vVbyHiT7vPmRorS+v9t2+N46ak=Xrew2cFf5+utC=tQ@mail.gmail.com> <8ef3ee-ba41-8e9e-4453-73736ff27783@google.com> <df5819aa-c79-6b10-4ce1-a45af118c0f3@google.com>
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There is no xas_pause(&xas) in collapse_file()'s main loop, at the points
where it does xas_unlock_irq(&xas) and then continues.

That would explain why, once two weeks ago and twice yesterday, I have
hit the VM_BUG_ON_PAGE(page != xas_load(&xas), page) since "mm/khugepaged:
fix iteration in collapse_file" removed the xas_set(&xas, index) just
before it: xas.xa_node could be left pointing to a stale node, if there
was concurrent activity on the file which transformed its xarray.

I tried inserting xas_pause()s, but then even bootup crashed on that
VM_BUG_ON_PAGE(): there appears to be a subtle "nextness" implicit in
xas_pause().

xas_next() and xas_pause() are good for use in simple loops, but not in
this one: xas_set() worked well until now, so use xas_set(&xas, index)
explicitly at the head of the loop; and change that VM_BUG_ON_PAGE() not
to need its own xas_set(), and not to interfere with the xa_state (which
would probably stop the crashes from xas_pause(), but I trust that less).

The user-visible effects of this bug (if VM_BUG_ONs are configured out)
would be data loss and data leak - potentially - though in practice I
expect it is more likely that a subsequent check (e.g. on mapping or on
nr_none) would notice an inconsistency, and just abandon the collapse.

Link: https://lore.kernel.org/linux-mm/f18e4b64-3f88-a8ab-56cc-d1f5f9c58d4@google.com/
Fixes: c8a8f3b4a95a ("mm/khugepaged: fix iteration in collapse_file")
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: <stable@vger.kernel.org>
---
 mm/khugepaged.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 3beb4ad2ee5e..78c8d5d8b628 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1937,9 +1937,9 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 		}
 	} while (1);
 
-	xas_set(&xas, start);
 	for (index = start; index < end; index++) {
-		page = xas_next(&xas);
+		xas_set(&xas, index);
+		page = xas_load(&xas);
 
 		VM_BUG_ON(index != xas.xa_index);
 		if (is_shmem) {
@@ -1954,7 +1954,6 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 						result = SCAN_TRUNCATED;
 						goto xa_locked;
 					}
-					xas_set(&xas, index + 1);
 				}
 				if (!shmem_charge(mapping->host, 1)) {
 					result = SCAN_FAIL;
@@ -2090,7 +2089,7 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 
 		xas_lock_irq(&xas);
 
-		VM_BUG_ON_PAGE(page != xas_load(&xas), page);
+		VM_BUG_ON_PAGE(page != xa_load(xas.xa, index), page);
 
 		/*
 		 * We control three references to the page:
-- 
2.35.3