Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> writes:
> Commit 73444bc4d8f9 ("mm, page_alloc: do not wake kswapd with zone lock
> held") moved wakeup_kswapd() from steal_suitable_fallback() to rmqueue()
> using ZONE_BOOSTED_WATERMARK flag.
>
> Only allocation contexts that include ALLOC_KSWAPD (which corresponds to
> __GFP_KSWAPD_RECLAIM) should wake kswapd, for callers are supposed to
> remove __GFP_KSWAPD_RECLAIM if trying to hold pgdat->kswapd_wait has a
> risk of deadlock. But since zone->flags is a shared variable, a thread
> doing !__GFP_KSWAPD_RECLAIM allocation request might observe this flag
> being set immediately after another thread doing __GFP_KSWAPD_RECLAIM
> allocation request set this flag, causing possibility of deadlock.
Sorry, I don't understand what is the deadlock here.
I checked commit 73444bc4d8f9 ("mm, page_alloc: do not wake kswapd with
zone lock held") and the corresponding mail thread. From the below
mail,
https://lore.kernel.org/all/20190107204627.GA25526@xxxxxxxxxxx/
commit 73444bc4d8f9 fixed a circular locking ordering as follows,
pi lock -> rq lock -> timer base lock -> zone lock -> wakeup lock
(kswapd_wait, fixed) -> pi lock
But I don't know what is the deadlock that your patch fixed. Can you
teach me on that?
Best Regards,
Huang, Ying
> Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Fixes: 73444bc4d8f9 ("mm, page_alloc: do not wake kswapd with zone lock held")
> ---
> Changes in v2:
> Check ALLOC_KSWAPD before checking ZONE_BOOSTED_WATERMARK and update
> description, suggested by Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>.
>
> mm/page_alloc.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 47421bedc12b..ecad680cec53 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3052,7 +3052,8 @@ struct page *rmqueue(struct zone *preferred_zone,
>
> out:
> /* Separate test+clear to avoid unnecessary atomics */
> - if (unlikely(test_bit(ZONE_BOOSTED_WATERMARK, &zone->flags))) {
> + if ((alloc_flags & ALLOC_KSWAPD) &&
> + unlikely(test_bit(ZONE_BOOSTED_WATERMARK, &zone->flags))) {
> clear_bit(ZONE_BOOSTED_WATERMARK, &zone->flags);
> wakeup_kswapd(zone, 0, 0, zone_idx(zone));
> }
