2013/8/19 Minchan Kim <minchan@xxxxxxxxxx>: > On Mon, Aug 19, 2013 at 10:17:38AM +0800, Bob Liu wrote: >> Hi Weijie, >> >> On 08/19/2013 12:14 AM, Weijie Yang wrote: >> > I found a few bugs in zswap when I review Linux-3.11-rc5, and I have >> > also some questions about it, described as following: >> > >> > BUG: >> > 1. A race condition when reclaim a page >> > when a handle alloced from zbud, zbud considers this handle is used >> > validly by upper(zswap) and can be a candidate for reclaim. >> > But zswap has to initialize it such as setting swapentry and addding >> > it to rbtree. so there is a race condition, such as: >> > thread 0: obtain handle x from zbud_alloc >> > thread 1: zbud_reclaim_page is called >> > thread 1: callback zswap_writeback_entry to reclaim handle x >> > thread 1: get swpentry from handle x (it is random value now) >> > thread 1: bad thing may happen >> > thread 0: initialize handle x with swapentry > > Nice catch! > >> >> Yes, this may happen potentially but in rare case. >> Because we have a LRU list for page frames, after Thread 0 called >> zbud_alloc the corresponding page will be add to the head of LRU >> list,While zbud_reclaim_page(Thread 1 called) is started from the tail >> of LRU list. >> >> > Of course, this situation almost never happen, it is a "theoretical >> > race condition" issue. > > But it's doable and we should prevent that although you feel it's rare > because system could go hang. When I look at the code, Why should zbud > have LRU logic instead of zswap? If I missed some history, sorry about that. > But at least to me, zbud is just allocator so it should have a role > to handle alloc/free object and how client of the allocator uses objects > depends on the upper layer so zbud should handle LRU. If so, we wouldn't > encounter this problem, either. > >> > >> > 2. Pollute swapcache data by add a pre-invalided swap page >> > when a swap_entry is invalidated, it will be reused by other anon >> > page. At the same time, zswap is reclaiming old page, pollute >> > swapcache of new page as a result, because old page and new page use >> > the same swap_entry, such as: >> > thread 1: zswap reclaim entry x >> > thread 0: zswap_frontswap_invalidate_page entry x >> > thread 0: entry x reused by other anon page >> > thread 1: add old data to swapcache of entry x >> >> I didn't get your idea here, why thread1 will add old data to entry x? >> >> > thread 0: swapcache of entry x is polluted >> > Of course, this situation almost never happen, it is another >> > "theoretical race condition" issue. > > Don't swapcache_prepare close the race? Yes, I made a mistake, there is not a race here. However, I find another bug here after my more careful review. It is not only "theoretical", it will happen really. as: thread 1: zswap reclaim entry x (get the refcount, but not call zswap_get_swap_cache_page yet) thread 0: zswap_frontswap_invalidate_page entry x (finished, entry x and its zbud is not freed as its refcount != 0) now, the swap_map[x] = 0 thread 1: zswap_get_swap_cache_page called, swapcache_prepare return -ENOENT because entry x is not used any more zswap_get_swap_cache_page return ZSWAP_SWAPCACHE_NOMEM zswap_writeback_entry do nothing except put refcount now, the memory of zswap_entry x leaks and its zpage become a zombie Best Regards, Weijie Yang -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>