Messages in this thread | | | Date | Thu, 15 Aug 2002 15:32:46 +0100 (BST) | From | Hugh Dickins <> | Subject | Re: 2.4.18(19) swapcache oops |
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On Thu, 15 Aug 2002 j-nomura@ce.jp.nec.com wrote: > > I'm using 2.4.18 kernel and suspect there are swapcache race. > I looked into 2.4.19 patch but could not find the fix to it.
I see a benign race but no oops.
> In the situation such as: > - two processes (process A and B) sharing memory space > - one of the pages in the space has been swapped out and > not remained in swapcache > - process A runs on cpu0, process B runs on cpu1 > > when process A reads the address corresponding to the page, > page fault occurs and the cpu0 reads swapped-out page into memory, > calls add_to_page_cache_unique() to add it to swapcache and then > calls lru_cache_add() to add it to lru list. > > If process B reads the same address at that time, cpu1 calls > do_swap_page() and lookup_swap_cache() may succeed before cpu0 > calls lru_cache_add() and cpu1 will set the page active by > following mark_page_accessed().
I agree that B may get to mark_page_accessed before A gets to lru_cache_add, but B will just SetPageReferenced. If there's a similar process C racing on the page too, its mark_page_accessed would call activate_page, but that will see !PageLRU and do nothing.
> lru_cache_add() checks if the page is active and if it is active, > it calls BUG().
It cannot be made PageActive until after lru_cache_add has set PageLRU.
Hugh
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