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Date:   Thu, 2 Apr 2020 09:36:12 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Daniel Jordan <daniel.m.jordan@oracle.com>
Cc:     Pavel Tatashin <pasha.tatashin@soleen.com>,
        linux-kernel@vger.kernel.org, akpm@linux-foundation.org,
        linux-mm@kvack.org, dan.j.williams@intel.com,
        shile.zhang@linux.alibaba.com, ktkhai@virtuozzo.com,
        david@redhat.com, jmorris@namei.org, sashal@kernel.org
Subject: Re: [PATCH] mm: initialize deferred pages with interrupts enabled
Message-ID: <20200402073612.GG22681@dhcp22.suse.cz>
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On Wed 01-04-20 16:08:55, Daniel Jordan wrote:
[...]
> From: Daniel Jordan <daniel.m.jordan@oracle.com>
> Date: Fri, 27 Mar 2020 17:29:05 -0400
> Subject: [PATCH] mm: call touch_nmi_watchdog() on max order boundaries in
>  deferred init
> 
> deferred_init_memmap() disables interrupts the entire time, so it calls
> touch_nmi_watchdog() periodically to avoid soft lockup splats.  Soon it
> will run with interrupts enabled, at which point cond_resched() should
> be used instead.
> 
> deferred_grow_zone() makes the same watchdog calls through code shared
> with deferred init but will continue to run with interrupts disabled, so
> it can't call cond_resched().
> 
> Pull the watchdog calls up to these two places to allow the first to be
> changed later, independently of the second.  The frequency reduces from
> twice per pageblock (init and free) to once per max order block.

This makes sense but I am not really sure this is necessary for the
stable backport.

> Signed-off-by: Daniel Jordan <daniel.m.jordan@oracle.com>

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
>  mm/page_alloc.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 212734c4f8b0..4cf18c534233 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1639,7 +1639,6 @@ static void __init deferred_free_pages(unsigned long pfn,
>  		} else if (!(pfn & nr_pgmask)) {
>  			deferred_free_range(pfn - nr_free, nr_free);
>  			nr_free = 1;
> -			touch_nmi_watchdog();
>  		} else {
>  			nr_free++;
>  		}
> @@ -1669,7 +1668,6 @@ static unsigned long  __init deferred_init_pages(struct zone *zone,
>  			continue;
>  		} else if (!page || !(pfn & nr_pgmask)) {
>  			page = pfn_to_page(pfn);
> -			touch_nmi_watchdog();
>  		} else {
>  			page++;
>  		}
> @@ -1813,8 +1811,10 @@ static int __init deferred_init_memmap(void *data)
>  	 * that we can avoid introducing any issues with the buddy
>  	 * allocator.
>  	 */
> -	while (spfn < epfn)
> +	while (spfn < epfn) {
>  		nr_pages += deferred_init_maxorder(&i, zone, &spfn, &epfn);
> +		touch_nmi_watchdog();
> +	}
>  zone_empty:
>  	pgdat_resize_unlock(pgdat, &flags);
>  
> @@ -1908,6 +1908,7 @@ deferred_grow_zone_locked(pg_data_t *pgdat, struct zone *zone,
>  		first_deferred_pfn = spfn;
>  
>  		nr_pages += deferred_init_maxorder(&i, zone, &spfn, &epfn);
> +		touch_nmi_watchdog();
>  
>  		/* We should only stop along section boundaries */
>  		if ((first_deferred_pfn ^ spfn) < PAGES_PER_SECTION)
> -- 
> 2.25.0
> 

-- 
Michal Hocko
SUSE Labs