On Sun, Jan 02, 2022 at 05:35:50PM -0800, Hugh Dickins wrote: > shmem_swapin_page()'s swap_free() has occasionally been generating > "_swap_info_get: Unused swap offset entry" messages. Usually that's > no worse than noise; but perhaps it indicates a worse case, when we > might there be freeing swap already reused by others. > > The multi-index xas_find_conflict() loop in shmem_add_to_page_cache() > did not allow for entry found NULL when expected to be non-NULL, so did > not catch that race when the swap has already been freed. > > The loop would not actually catch a realistic conflict which the single > check does not catch, so revert it back to the single check. I think what led to the loop was concern for the xa_state if trying to find a swap entry that's smaller than the size of the folio. So yes, the loop was expected to execute twice, but I didn't consider the case where we were looking for something non-NULL and actually found NULL. So should we actually call xas_find_conflict() twice (if we're looking for something non-NULL), and check that we get @expected, followed by NULL?
