Re: [BUG REPORT] ZSWAP: theoretical race condition issues

[Weijie Yang]

I think I find a new issue, for integrity of this mail thread, I reply
to this mail.

It is a concurrence issue either, when duplicate store and reclaim
concurrentlly.

zswap entry x with offset A is already stored in zswap backend.
Consider the following scenario:

thread 0: reclaim entry x (get refcount, but not call zswap_get_swap_cache_page)

thread 1: store new page with the same offset A, alloc a new zswap entry y.
  store finished. shrink_page_list() call __remove_mapping(), and now
it is not in swap_cache

thread 0: zswap_get_swap_cache_page called. old page data is added to swap_cache

Now, swap cache has old data rather than new data for offset A.
error will happen If do_swap_page() get page from swap_cache.
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