Messages in this thread | | | Date | Wed, 25 Sep 2013 18:02:15 +0800 | Subject | Re: [BUG REPORT] ZSWAP: theoretical race condition issues | From | Bob Liu <> |
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On Wed, Sep 25, 2013 at 5:33 PM, Weijie Yang <weijie.yang.kh@gmail.com> wrote: > On Wed, Sep 25, 2013 at 4:31 PM, Bob Liu <lliubbo@gmail.com> wrote: >> On Wed, Sep 25, 2013 at 4:09 PM, Weijie Yang <weijie.yang.kh@gmail.com> wrote: >>> 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 >>> >> >> But I don't think swap layer will call zswap with the same offset A. > > 1. store page of offset A in zswap > 2. some time later, pagefault occur, load page data from zswap. > But notice that zswap entry x is still in zswap because it is not
Sorry I didn't notice that zswap_frontswap_load() doesn't call rb_erase().
> frontswap_tmem_exclusive_gets_enabled. > this page is with PageSwapCache(page) and page_private(page) = entry.val > 3. change this page data, and it become dirty > 4. some time later again, swap this page on the same offset A. > > so, a duplicate store happens. >
Then I think we should erase the entry from rbtree in zswap_frontswap_load(). After the page is decompressed and loaded from zswap, still storing the compressed data in zswap is meanless.
-- Regards, --Bob
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