Re: [PATCH V3 1/2] mm: avoid marking swap cached page as lazyfree

[Minchan Kim <minchan@kernel.org>]

On Tue, Sep 26, 2017 at 10:26:25AM -0700, Shaohua Li wrote:
> From: Shaohua Li <shli@fb.com>
> 
> MADV_FREE clears pte dirty bit and then marks the page lazyfree (clear
> SwapBacked). There is no lock to prevent the page is added to swap cache
> between these two steps by page reclaim. Page reclaim could add the page
> to swap cache and unmap the page. After page reclaim, the page is added
> back to lru. At that time, we probably start draining per-cpu pagevec
> and mark the page lazyfree. So the page could be in a state with
> SwapBacked cleared and PG_swapcache set. Next time there is a refault in
> the virtual address, do_swap_page can find the page from swap cache but
> the page has PageSwapCache false because SwapBacked isn't set, so
> do_swap_page will bail out and do nothing. The task will keep running
> into fault handler.

With new description, I got why you want to seperate this. Yub, it should
be separated. Sorry for the noise. What I was missing is PageSwapCache's
change which checked PG_swapbacked as well as PG_swapcache. I didn't 
notice that the change.

Acked-by: Minchan Kim <minchan@kernel.org>

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