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Message-ID: <b6ba16ce-4849-d32c-68fe-07a15aaf9d9c@I-love.SAKURA.ne.jp>
Date:   Sat, 29 Jul 2023 14:31:01 +0900
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Subject: Re: [PATCH v2 1/2] seqlock: Do the lockdep annotation before locking
 in do_write_seqcount_begin_nested()
Content-Language: en-US
To:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Michal Hocko <mhocko@suse.com>
Cc:     Petr Mladek <pmladek@suse.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        "Luis Claudio R. Goncalves" <lgoncalv@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Ingo Molnar <mingo@redhat.com>,
        John Ogness <john.ogness@linutronix.de>,
        Mel Gorman <mgorman@techsingularity.net>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Waiman Long <longman@redhat.com>, Will Deacon <will@kernel.org>
References: <20230623171232.892937-1-bigeasy@linutronix.de>
 <20230623171232.892937-2-bigeasy@linutronix.de>
 <d9b7c170-ed0d-5d37-e099-20d233115943@I-love.SAKURA.ne.jp>
 <20230626081254.XmorFrhs@linutronix.de> <ZJmkPuqpW-wQAyNz@alley>
 <a1c559b7-335e-5401-d167-301c5b1cd312@I-love.SAKURA.ne.jp>
 <20230727151029.e_M9bi8N@linutronix.de>
From:   Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
In-Reply-To: <20230727151029.e_M9bi8N@linutronix.de>
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Precedence: bulk

On 2023/07/28 0:10, Sebastian Andrzej Siewior wrote:
> On 2023-06-28 21:14:16 [+0900], Tetsuo Handa wrote:
>>> Anyway, please do not do this change only because of printk().
>>> IMHO, the current ordering is more logical and the printk() problem
>>> should be solved another way.
>>
>> Then, since [PATCH 1/2] cannot be applied, [PATCH 2/2] is automatically
>> rejected.
> 
> My understanding is that this patch gets applied and your objection will
> be noted.

My preference is that zonelist_update_seq is not checked by !__GFP_DIRECT_RECLAIM
allocations, which is a low-hanging fruit towards GFP_LOCKLESS mentioned at
https://lkml.kernel.org/r/ZG3+l4qcCWTPtSMD@dhcp22.suse.cz and
https://lkml.kernel.org/r/ZJWWpGZMJIADQvRS@dhcp22.suse.cz .

Maybe we can defer checking zonelist_update_seq till retry check like below,
for this is really an infrequent event.

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 7d3460c7a480..2f7b82af2590 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3642,22 +3642,27 @@ EXPORT_SYMBOL_GPL(fs_reclaim_release);
  * retries the allocation if zonelist changes. Writer side is protected by the
  * embedded spin_lock.
  */
-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);
+		return data_race(READ_ONCE(zonelist_update_seq));
 
 	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)) {
+		unsigned int seq2;
+
+		smp_rmb();
+		seq2 = data_race(READ_ONCE(zonelist_update_seq));
+		return unlikely(seq != seq2 || (seq2 & 1));
+	}
 
-	return seq;
+	return 0;
 }
 
 /* Perform direct synchronous page reclaim */
@@ -4146,7 +4151,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 	 * a unnecessary OOM kill.
 	 */
 	if (check_retry_cpuset(cpuset_mems_cookie, ac) ||
-	    check_retry_zonelist(zonelist_iter_cookie))
+	    check_retry_zonelist(gfp_mask, zonelist_iter_cookie))
 		goto restart;
 
 	/* Reclaim has failed us, start killing things */
@@ -4172,7 +4177,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 	 * a unnecessary OOM kill.
 	 */
 	if (check_retry_cpuset(cpuset_mems_cookie, ac) ||
-	    check_retry_zonelist(zonelist_iter_cookie))
+	    check_retry_zonelist(gfp_mask, zonelist_iter_cookie))
 		goto restart;
 
 	/*
@@ -5136,22 +5141,12 @@ 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);
+	spin_lock_irqsave(&lock, flags);
+	data_race(zonelist_update_seq++);
+	smp_wmb();
 
 #ifdef CONFIG_NUMA
 	memset(node_load, 0, sizeof(node_load));
@@ -5188,9 +5183,9 @@ static void __build_all_zonelists(void *data)
 #endif
 	}
 
-	write_sequnlock(&zonelist_update_seq);
-	printk_deferred_exit();
-	local_irq_restore(flags);
+	smp_wmb();
+	data_race(zonelist_update_seq++);
+	spin_unlock_irqrestore(&lock, flags);
 }
 
 static noinline void __init