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swap: fix races exposed by swap discard
The previous patch can expose races, according to Hugh: swapoff was sometimes failing with "Cannot allocate memory", coming from try_to_unuse()'s -ENOMEM: it needs to allow for swap_duplicate() failing on a free entry temporarily SWAP_MAP_BAD while being discarded. We should use ACCESS_ONCE() there, and whenever accessing swap_map locklessly; but rather than peppering it throughout try_to_unuse(), just declare *swap_map with volatile. try_to_unuse() is accustomed to *swap_map going down racily, but not necessarily to it jumping up from 0 to SWAP_MAP_BAD: we'll be safer to prevent that transition once SWP_WRITEOK is switched off, when it's a waste of time to issue discards anyway (swapon can do a whole discard). Another issue is: In swapin_readahead(), read_swap_cache_async() can read a bad swap entry, because we don't check if readahead swap entry is bad. This doesn't break anything but such swapin page is wasteful and can only be freed at page reclaim. We should avoid read such swap entry. And in discard, we mark swap entry SWAP_MAP_BAD and then switch it to normal when discard is finished. If readahead reads such swap entry, we have the same issue, so we much check if swap entry is bad too. Thanks Hugh to inspire swapin_readahead could use bad swap entry. [include Hugh's patch 'swap: fix swapoff ENOMEMs from discard'] Signed-off-by:Shaohua Li <shli@fusionio.com> Signed-off-by: