From: Hugh Dickins <hughd@xxxxxxxxxx> commit e8c716bc6812202ccf4ce0f0bad3428b794fb39c upstream. 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@xxxxxxxxxx/ Fixes: c8a8f3b4a95a ("mm/khugepaged: fix iteration in collapse_file") Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx> Cc: stable@xxxxxxxxxx Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx> Cc: David Stevens <stevensd@xxxxxxxxxxxx> Cc: Peter Xu <peterx@xxxxxxxxxx> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- mm/khugepaged.c | 7 +++---- 1 file changed, 3 insertions(+), 4 deletions(-) --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -1918,9 +1918,9 @@ static int collapse_file(struct mm_struc } } 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) { @@ -1935,7 +1935,6 @@ static int collapse_file(struct mm_struc result = SCAN_TRUNCATED; goto xa_locked; } - xas_set(&xas, index + 1); } if (!shmem_charge(mapping->host, 1)) { result = SCAN_FAIL; @@ -2071,7 +2070,7 @@ static int collapse_file(struct mm_struc 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:
