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

[Tetsuo Handa]

On 2023/08/10 18:58, Tetsuo Handa wrote:
> If __build_all_zonelists() can run without being switched to other threads
> (except interrupt handlers), I consider that this approach works.

If there is no way to make sure that the section between
write_seqlock(&zonelist_update_seq) and write_sequnlock(&zonelist_update_seq)
runs without context switching (interrupts handlers are fine), something like
below could be used in order to keep

  spin_lock(s->lock);
  spin_unlock(s->lock);

away from seqprop_sequence() from atomic allocations. But I think that looses
the reason to replace read_seqbegin() with raw_seqcount_begin(); that will be
essentially the same with
https://lkml.kernel.org/r/dfdb9da6-ca8f-7a81-bfdd-d74b4c401f11@xxxxxxxxxxxxxxxxxxx .



diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 7d3460c7a480..f2f79caab2cf 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3644,20 +3644,20 @@ EXPORT_SYMBOL_GPL(fs_reclaim_release);
  */
 static DEFINE_SEQLOCK(zonelist_update_seq);
 
-static unsigned int zonelist_iter_begin(void)
+static unsigned int zonelist_iter_begin(gfp_t gfp)
 {
-	if (IS_ENABLED(CONFIG_MEMORY_HOTREMOVE))
-		return read_seqbegin(&zonelist_update_seq);
+	if (IS_ENABLED(CONFIG_MEMORY_HOTREMOVE) && (gfp & __GFP_DIRECT_RECLAIM))
+		return data_race(raw_seqcount_begin(&zonelist_update_seq.seqcount));
 
 	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))
+		return data_race(read_seqcount_retry(&zonelist_update_seq.seqcount, seq));
 
-	return seq;
+	return 0;
 }
 
 /* Perform direct synchronous page reclaim */
@@ -3968,7 +3968,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 	no_progress_loops = 0;
 	compact_priority = DEF_COMPACT_PRIORITY;
 	cpuset_mems_cookie = read_mems_allowed_begin();
-	zonelist_iter_cookie = zonelist_iter_begin();
+	zonelist_iter_cookie = zonelist_iter_begin(gfp_mask);
 
 	/*
 	 * The fast path uses conservative alloc_flags to succeed only until
@@ -4146,7 +4146,7 @@ __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 +4172,7 @@ __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;
 
 	/*
@@ -5138,20 +5138,7 @@ static void __build_all_zonelists(void *data)
 	pg_data_t *self = data;
 	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);
+	write_seqlock_irqsave(&zonelist_update_seq, flags);
 
 #ifdef CONFIG_NUMA
 	memset(node_load, 0, sizeof(node_load));
@@ -5188,9 +5175,7 @@ static void __build_all_zonelists(void *data)
 #endif
 	}
 
-	write_sequnlock(&zonelist_update_seq);
-	printk_deferred_exit();
-	local_irq_restore(flags);
+	write_sequnlock_irqrestore(&zonelist_update_seq, flags);
 }
 
 static noinline void __init