Re: [PATCH v2] mm/page_alloc: don't check zonelist_update_seq from atomic allocations

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2023-08-09 20:03:00 [+0900], Tetsuo Handa wrote:
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 7d3460c7a480..5557d9a2ff2c 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3638,26 +3638,44 @@ EXPORT_SYMBOL_GPL(fs_reclaim_release);
…
> -static DEFINE_SEQLOCK(zonelist_update_seq);
> +static unsigned int zonelist_update_seq;
>  
>  static unsigned int zonelist_iter_begin(void)
>  {
>  	if (IS_ENABLED(CONFIG_MEMORY_HOTREMOVE))
> -		return read_seqbegin(&zonelist_update_seq);
> +		/* See comment above. */
> +		return data_race(READ_ONCE(zonelist_update_seq));

This is open coded raw_read_seqcount() while it should have been
raw_seqcount_begin().

>  	return 0;
>  }
>  
> -static unsigned int check_retry_zonelist(unsigned int seq)
> +static unsigned int check_retry_zonelist(gfp_t gfp, unsigned int seq)
>  {
> -	if (IS_ENABLED(CONFIG_MEMORY_HOTREMOVE))
> -		return read_seqretry(&zonelist_update_seq, seq);
> +	if (IS_ENABLED(CONFIG_MEMORY_HOTREMOVE) && (gfp & __GFP_DIRECT_RECLAIM)) {
> +		/* See comment above. */
> +		unsigned int seq2 = data_race(READ_ONCE(zonelist_update_seq));
>  
> -	return seq;
> +		/*
> +		 * "seq != seq2" indicates that __build_all_zonelists() has
> +		 * started or has finished rebuilding zonelists, hence retry.
> +		 * "seq == seq2 && (seq2 & 1)" indicates that
> +		 * __build_all_zonelists() is still rebuilding zonelists
> +		 * with context switching disabled, hence retry.
> +		 * "seq == seq2 && !(seq2 & 1)" indicates that
> +		 * __build_all_zonelists() did not rebuild zonelists, hence
> +		 * no retry.
> +		 */
> +		return unlikely(seq != seq2 || (seq2 & 1));

open coded read_seqcount_retry().

> +	}
> +
> +	return 0;
>  }
>  
>  /* Perform direct synchronous page reclaim */
> @@ -5136,22 +5154,17 @@ static void __build_all_zonelists(void *data)
>  	int nid;
>  	int __maybe_unused cpu;
>  	pg_data_t *self = data;
> +	static DEFINE_SPINLOCK(lock);
>  	unsigned long flags;
>  
> -	/*
> -	 * Explicitly disable this CPU's interrupts before taking seqlock
> -	 * to prevent any IRQ handler from calling into the page allocator
> -	 * (e.g. GFP_ATOMIC) that could hit zonelist_iter_begin and livelock.
> -	 */
> -	local_irq_save(flags);
> -	/*
> -	 * Explicitly disable this CPU's synchronous printk() before taking
> -	 * seqlock to prevent any printk() from trying to hold port->lock, for
> -	 * tty_insert_flip_string_and_push_buffer() on other CPU might be
> -	 * calling kmalloc(GFP_ATOMIC | __GFP_NOWARN) with port->lock held.
> -	 */
> -	printk_deferred_enter();
> -	write_seqlock(&zonelist_update_seq);
> +#ifdef CONFIG_PREEMPT_RT
> +	migrate_disable()
> +#endif

There is no justification/ explanation why migrate_disable() here is
needed on PREEMPT_RT and I don't see one.

There are two changes here:
- The replacement of seqlock_t with something open coded 
- Logic change when a retry is needed (the gfp mask is considered).

I am not a big fan of open coding things especially when not needed and
then there is this ifdef which is not needed as well. I don't comment on
the logic change.

Can we please put an end to this?

> +	/* Serialize increments of zonelist_update_seq. */
> +	spin_lock_irqsave(&lock, flags);
> +	zonelist_update_seq++;
> +	/* Tell check_retry_zonelist() that we started rebuilding zonelists. */
> +	smp_wmb();
>  
>  #ifdef CONFIG_NUMA
>  	memset(node_load, 0, sizeof(node_load));

Sebastian