Re: 2.4.18(19) swapcache oops

[Hugh Dickins]

On Thu, 15 Aug 2002 [e-mail] 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