Re: [PATCH stable-4.9 2/2] mm, vmscan: consider eligible zones in get_scan_count

[MegaBrutal]

Hi Michal,

I'm over a very long kernel bisection, and if I made no mistake in
testing commits, this patch fixes a kernel bug which affects my HP
Compaq dc5800 machine with 32 bit Ubuntu OS.

The bug manifests itself with "NMI watchdog: BUG: soft lockup - CPU#0
stuck for 23s! [kswapd0:38]" messages in 4.8 kernels, and "page
allocation stalls for 47608ms, order:1,
mode:0x17000c0(GFP_KERNEL_ACCOUNT|__GFP_NOTRACK)" in 4.10 kernels up
to this commit.

Michal, can you confirm that this patch may fix issues like the ones I
encountered? If so, I'll try to get the Ubuntu kernel staff to
backport this commit to Yakkety's 4.8 kernel. On the other hand, I
can't seem to be able to backport this commit to 4.8 with "git
cherry-pick", so maybe I need to wait for your tweaks you mentioned.

Anyway, thank you very much for the fix!


Regards,
MegaBrutal



2017-02-28 16:11 GMT+01:00 Michal Hocko <mhocko@xxxxxxxxxx>:
>
> From: Michal Hocko <mhocko@xxxxxxxx>
>
> commit 71ab6cfe88dcf9f6e6a65eb85cf2bda20a257682 upstream.
>
> get_scan_count() considers the whole node LRU size when
>
>  - doing SCAN_FILE due to many page cache inactive pages
>  - calculating the number of pages to scan
>
> In both cases this might lead to unexpected behavior especially on 32b
> systems where we can expect lowmem memory pressure very often.
>
> A large highmem zone can easily distort SCAN_FILE heuristic because
> there might be only few file pages from the eligible zones on the node
> lru and we would still enforce file lru scanning which can lead to
> trashing while we could still scan anonymous pages.
>
> The later use of lruvec_lru_size can be problematic as well.  Especially
> when there are not many pages from the eligible zones.  We would have to
> skip over many pages to find anything to reclaim but shrink_node_memcg
> would only reduce the remaining number to scan by SWAP_CLUSTER_MAX at
> maximum.  Therefore we can end up going over a large LRU many times
> without actually having chance to reclaim much if anything at all.  The
> closer we are out of memory on lowmem zone the worse the problem will
> be.
>
> Fix this by filtering out all the ineligible zones when calculating the
> lru size for both paths and consider only sc->reclaim_idx zones.
>
> The patch would need to be tweaked a bit to apply to 4.10 and older but
> I will do that as soon as it hits the Linus tree in the next merge
> window.
>
> Link: http://lkml.kernel.org/r/20170117103702.28542-3-mhocko@xxxxxxxxxx
> Fixes: b2e18757f2c9 ("mm, vmscan: begin reclaiming pages on a per-node basis")
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> Tested-by: Trevor Cordes <trevor@xxxxxxxxxxxxx>
> Acked-by: Minchan Kim <minchan@xxxxxxxxxx>
> Acked-by: Hillf Danton <hillf.zj@xxxxxxxxxxxxxxx>
> Acked-by: Mel Gorman <mgorman@xxxxxxx>
> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>    [4.8+]
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> ---
>  mm/vmscan.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index cd516c632e8f..30a88b945a44 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2205,7 +2205,7 @@ static void get_scan_count(struct lruvec *lruvec, struct mem_cgroup *memcg,
>          * system is under heavy pressure.
>          */
>         if (!inactive_list_is_low(lruvec, true, sc) &&
> -           lruvec_lru_size(lruvec, LRU_INACTIVE_FILE, MAX_NR_ZONES) >> sc->priority) {
> +           lruvec_lru_size(lruvec, LRU_INACTIVE_FILE, sc->reclaim_idx) >> sc->priority) {
>                 scan_balance = SCAN_FILE;
>                 goto out;
>         }
> @@ -2272,7 +2272,7 @@ static void get_scan_count(struct lruvec *lruvec, struct mem_cgroup *memcg,
>                         unsigned long size;
>                         unsigned long scan;
>
> -                       size = lruvec_lru_size(lruvec, lru, MAX_NR_ZONES);
> +                       size = lruvec_lru_size(lruvec, lru, sc->reclaim_idx);
>                         scan = size >> sc->priority;
>
>                         if (!scan && pass && force_scan)
> --
> 2.11.0
>