diff options
| author | Hugh Dickins <hughd@google.com> | 2023-06-28 21:31:35 -0700 |
|---|---|---|
| committer | Linus Torvalds <torvalds@linux-foundation.org> | 2023-06-29 09:41:45 -0700 |
| commit | e8c716bc6812202ccf4ce0f0bad3428b794fb39c (patch) | |
| tree | 282d189732b298f2a80938ce6c5249b2575d49d1 | |
| parent | 9471f1f2f50282b9e8f59198ec6bb738b4ccc009 (diff) | |
mm/khugepaged: fix regression in collapse_file()
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@kernel.org
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: David Stevens <stevensd@chromium.org>
Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
| -rw-r--r-- | mm/khugepaged.c | 7 |
1 files 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: |