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

[Johannes Weiner <hannes@cmpxchg.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.

The patch lgtm, but for the changelog it probably makes sense to start
with the user-visible behavior, i.e. the endlessly looping swap fault
handler because it thinks it's racing with the swap slot being freed.

Makes it easier for other distro/vendor people to identify this for
backporting.

On that note, I think this should go into 4.13 and be tagged for 4.12
stable.

> Reported-and-tested-by: Artem Savkov <asavkov@redhat.com>
> Fix: 802a3a92ad7a(mm: reclaim MADV_FREE pages)
> Signed-off-by: Shaohua Li <shli@fb.com>
> Cc: stable@vger.kernel.org
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Hillf Danton <hillf.zj@alibaba-inc.com>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Mel Gorman <mgorman@techsingularity.net>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Reviewed-by: Rik van Riel <riel@redhat.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

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