linux-kernel - Re: [v5 1/2] mm: disable interrupts while initializing deferred pages

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Message-Id: <20180312130410.e2fce8e5e38bc2086c7fd924@linux-foundation.org>
Date:   Mon, 12 Mar 2018 13:04:10 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Pavel Tatashin <pasha.tatashin@...cle.com>
Cc:     steven.sistare@...cle.com, daniel.m.jordan@...cle.com,
        m.mizuma@...fujitsu.com, mhocko@...e.com, catalin.marinas@....com,
        takahiro.akashi@...aro.org, gi-oh.kim@...fitbricks.com,
        heiko.carstens@...ibm.com, baiyaowei@...s.chinamobile.com,
        richard.weiyang@...il.com, paul.burton@...s.com,
        miles.chen@...iatek.com, vbabka@...e.cz, mgorman@...e.de,
        hannes@...xchg.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [v5 1/2] mm: disable interrupts while initializing deferred
 pages

On Fri,  9 Mar 2018 17:08:06 -0500 Pavel Tatashin <pasha.tatashin@...cle.com> wrote:

> Vlastimil Babka reported about a window issue during which when deferred
> pages are initialized, and the current version of on-demand initialization
> is finished, allocations may fail.  While this is highly unlikely scenario,
> since this kind of allocation request must be large, and must come from
> interrupt handler, we still want to cover it.
> 
> We solve this by initializing deferred pages with interrupts disabled, and
> holding node_size_lock spin lock while pages in the node are being
> initialized. The on-demand deferred page initialization that comes later
> will use the same lock, and thus synchronize with deferred_init_memmap().
> 
> It is unlikely for threads that initialize deferred pages to be
> interrupted.  They run soon after smp_init(), but before modules are
> initialized, and long before user space programs. This is why there is no
> adverse effect of having these threads running with interrupts disabled.
> 
> ...
>
> --- a/include/linux/memory_hotplug.h
> +++ b/include/linux/memory_hotplug.h
>  
> +#if defined(CONFIG_MEMORY_HOTPLUG) || defined(CONFIG_DEFERRED_STRUCT_PAGE_INIT)
> +/*
> + * pgdat resizing functions
> + */
> +static inline
> +void pgdat_resize_lock(struct pglist_data *pgdat, unsigned long *flags)
> +{
> +	spin_lock_irqsave(&pgdat->node_size_lock, *flags);
> +}
> +static inline
> +void pgdat_resize_unlock(struct pglist_data *pgdat, unsigned long *flags)
> +{
> +	spin_unlock_irqrestore(&pgdat->node_size_lock, *flags);
> +}
> +static inline
> +void pgdat_resize_init(struct pglist_data *pgdat)
> +{
> +	spin_lock_init(&pgdat->node_size_lock);
> +}
> +
> +/* Disable interrupts and save previous IRQ state in flags before locking */
> +static inline
> +void pgdat_resize_lock_irq(struct pglist_data *pgdat, unsigned long *flags)
> +{
> +	unsigned long tmp_flags;
> +
> +	local_irq_save(*flags);
> +	local_irq_disable();
> +	pgdat_resize_lock(pgdat, &tmp_flags);
> +}

As far as I can tell, this ugly-looking thing is identical to
pgdat_resize_lock().

> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1506,7 +1506,6 @@ static void __init deferred_free_pages(int nid, int zid, unsigned long pfn,
>  		} else if (!(pfn & nr_pgmask)) {
>  			deferred_free_range(pfn - nr_free, nr_free);
>  			nr_free = 1;
> -			cond_resched();
>  		} else {
>  			nr_free++;

And how can we simply remove these cond_resched()s?  I assume this is
being done because interrupts are now disabled?  But those were there
for a reason, weren't they?