linux-kernel - Re: [PATCH v2 2/2] mm: initialize deferred pages with interrupts enabled

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Date:   Thu, 2 Apr 2020 09:38:54 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Pavel Tatashin <pasha.tatashin@...een.com>,
        linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        mhocko@...e.com, linux-mm@...ck.org, dan.j.williams@...el.com,
        shile.zhang@...ux.alibaba.com, daniel.m.jordan@...cle.com,
        ktkhai@...tuozzo.com, jmorris@...ei.org, sashal@...nel.org,
        vbabka@...e.cz
Subject: Re: [PATCH v2 2/2] mm: initialize deferred pages with interrupts
 enabled

> +	/*
> +	 * Once we unlock here, the zone cannot be grown anymore, thus if an
> +	 * interrupt thread must allocate this early in boot, zone must be
> +	 * pre-grown prior to start of deferred page initialization.
> +	 */
> +	pgdat_resize_unlock(pgdat, &flags);
> +
>  	/* Only the highest zone is deferred so find it */
>  	for (zid = 0; zid < MAX_NR_ZONES; zid++) {
>  		zone = pgdat->node_zones + zid;
> @@ -1809,11 +1816,9 @@ static int __init deferred_init_memmap(void *data)
>  	 */
>  	while (spfn < epfn) {
>  		nr_pages += deferred_init_maxorder(&i, zone, &spfn, &epfn);
> -		touch_nmi_watchdog();
> +		cond_resched();

I do wonder if this change is strictly required in this patch (IOW, if
we could keep calling touch_nmi_watchdog() also without holding a spinlock)

Anyhow, it's the right thing to do.

>  	}
>  zone_empty:
> -	pgdat_resize_unlock(pgdat, &flags);
> -
>  	/* Sanity check that the next zone really is unpopulated */
>  	WARN_ON(++zid < MAX_NR_ZONES && populated_zone(++zone));
>  
> @@ -1855,17 +1860,6 @@ deferred_grow_zone(struct zone *zone, unsigned int order)
>  
>  	pgdat_resize_lock(pgdat, &flags);
>  
> -	/*
> -	 * If deferred pages have been initialized while we were waiting for
> -	 * the lock, return true, as the zone was grown.  The caller will retry
> -	 * this zone.  We won't return to this function since the caller also
> -	 * has this static branch.
> -	 */
> -	if (!static_branch_unlikely(&deferred_pages)) {
> -		pgdat_resize_unlock(pgdat, &flags);
> -		return true;
> -	}
> -
>  	/*
>  	 * If someone grew this zone while we were waiting for spinlock, return
>  	 * true, as there might be enough pages already.
> 


I think we should also look into cleaning up deferred_grow_zone( next),
we still have that touch_nmi_watchdog() in there. We should rework
locking. (I think Michal requested that as well)

For now, this seems to survive my basic testing (RCU stalls gone)

-- 
Thanks,

David / dhildenb