linux-kernel - Re: [PATCH 1/4] mm/mm_init: use deferred_init_memmap_chunk() in deferred_grow

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Message-ID: <20250819235158.mgei7l4yraheech4@master>
Date: Tue, 19 Aug 2025 23:51:58 +0000
From: Wei Yang <richard.weiyang@...il.com>
To: Mike Rapoport <rppt@...nel.org>
Cc: Wei Yang <richard.weiyang@...il.com>, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Bill Wendling <morbo@...gle.com>,
	Daniel Jordan <daniel.m.jordan@...cle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Miguel Ojeda <ojeda@...nel.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
	linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH 1/4] mm/mm_init: use deferred_init_memmap_chunk() in
 deferred_grow_zone()

On Tue, Aug 19, 2025 at 01:54:46PM +0300, Mike Rapoport wrote:
>On Tue, Aug 19, 2025 at 09:52:23AM +0000, Wei Yang wrote:
>> Hi, Mike
>> 
>> After going through the code again, I have some trivial thoughts to discuss
>> with you. If not right, please let me know.
>> 
>> On Mon, Aug 18, 2025 at 09:46:12AM +0300, Mike Rapoport wrote:
>> [...]
>> > bool __init deferred_grow_zone(struct zone *zone, unsigned int order)
>> > {
>> >-	unsigned long nr_pages_needed = ALIGN(1 << order, PAGES_PER_SECTION);
>> >+	unsigned long nr_pages_needed = SECTION_ALIGN_UP(1 << order);
>> > 	pg_data_t *pgdat = zone->zone_pgdat;
>> > 	unsigned long first_deferred_pfn = pgdat->first_deferred_pfn;
>> > 	unsigned long spfn, epfn, flags;
>> > 	unsigned long nr_pages = 0;
>> >-	u64 i = 0;
>> > 
>> > 	/* Only the last zone may have deferred pages */
>> > 	if (zone_end_pfn(zone) != pgdat_end_pfn(pgdat))
>> >@@ -2262,37 +2272,26 @@ bool __init deferred_grow_zone(struct zone *zone, unsigned int order)
>> > 		return true;
>> > 	}
>> 
>> In the file above this line, there is a compare between first_deferred_pfn and
>> its original value after grab pgdat_resize_lock.
>
>Do you mean this one:
>
>	if (first_deferred_pfn != pgdat->first_deferred_pfn) {
>		pgdat_resize_unlock(pgdat, &flags);
>		return true;
>	}
> 

Yes.

I am thinking something like this:

 	if (first_deferred_pfn != pgdat->first_deferred_pfn || 
	    first_deferred_pfn == ULONG_MAX)

This means

  * someone else has grow zone before we grab the lock
  * or the whole zone has already been initialized

>> I am thinking to compare first_deferred_pfn with ULONG_MAX, as it compared in
>> deferred_init_memmap(). This indicate this zone has already been initialized
>> totally.
>
>It may be another CPU ran deferred_grow_zone() and won the race for resize
>lock. Then pgdat->first_deferred_pfn will be larger than
>first_deferred_pfn, but still not entire zone would be initialized.
> 
>> Current code guard this by spfn < zone_end_pfn(zone). Maybe a check ahead
>> would be more clear?
>
>Not sure I follow you here. The check that we don't pass zone_end_pfn is
>inside the loop for every section we initialize.
> 

In case the zone has been initialized totally, first_deferred_pfn = ULONG_MAX.

Then we come to the loop with initial state:

    spfn = ULONG_MAX
    epfn = 0 (which is wrap around)

And loop condition check (spfn < zone_end_pfn(zone)) is false, so the loop is
skipped. This is how we handle a fully initialized zone now.

Would this be a little un-common?

>> > 
>> >-	/* If the zone is empty somebody else may have cleared out the zone */
>> >-	if (!deferred_init_mem_pfn_range_in_zone(&i, zone, &spfn, &epfn,
>> >-						 first_deferred_pfn)) {
>> >-		pgdat->first_deferred_pfn = ULONG_MAX;
>> >-		pgdat_resize_unlock(pgdat, &flags);
>> >-		/* Retry only once. */
>> >-		return first_deferred_pfn != ULONG_MAX;
>> >+	/*
>> >+	 * Initialize at least nr_pages_needed in section chunks.
>> >+	 * If a section has less free memory than nr_pages_needed, the next
>> >+	 * section will be also initalized.

Nit, one typo here. s/initalized/initialized/

>> >+	 * Note, that it still does not guarantee that allocation of order can
>> >+	 * be satisfied if the sections are fragmented because of memblock
>> >+	 * allocations.
>> >+	 */
>> >+	for (spfn = first_deferred_pfn, epfn = SECTION_ALIGN_UP(spfn + 1);

-- 
Wei Yang
Help you, Help me