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

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Message-ID: <20250819095223.ckjdsii4gc6u4nec@master>
Date: Tue, 19 Aug 2025 09:52:23 +0000
From: Wei Yang <richard.weiyang@...il.com>
To: Mike Rapoport <rppt@...nel.org>
Cc: 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()

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.

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.

Current code guard this by spfn < zone_end_pfn(zone). Maybe a check ahead
would be more clear?

> 
>-	/* 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.
>+	 * 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);

I am expecting first_deferred_pfn is section aligned. So epfn += PAGES_PER_SECTION
is fine?

Maybe I missed something.

>+	     nr_pages < nr_pages_needed && spfn < zone_end_pfn(zone);
>+	     spfn = epfn, epfn += PAGES_PER_SECTION) {
>+		nr_pages += deferred_init_memmap_chunk(spfn, epfn, zone);
> 	}
> 
> 	/*
>-	 * Initialize and free pages in MAX_PAGE_ORDER sized increments so
>-	 * that we can avoid introducing any issues with the buddy
>-	 * allocator.
>+	 * There were no pages to initialize and free which means the zone's
>+	 * memory map is completely initialized.
> 	 */
>-	while (spfn < epfn) {
>-		/* update our first deferred PFN for this section */
>-		first_deferred_pfn = spfn;
>-
>-		nr_pages += deferred_init_maxorder(&i, zone, &spfn, &epfn);
>-		touch_nmi_watchdog();
>-
>-		/* We should only stop along section boundaries */
>-		if ((first_deferred_pfn ^ spfn) < PAGES_PER_SECTION)
>-			continue;
>-
>-		/* If our quota has been met we can stop here */
>-		if (nr_pages >= nr_pages_needed)
>-			break;
>-	}
>+	pgdat->first_deferred_pfn = nr_pages ? spfn : ULONG_MAX;

If we come here because spfn >= zone_end_pfn(zone), first_deferred_pfn is left
a "valid" value and deferred_init_memmap() will try to do its job. But
actually nothing left to initialize.

For this case, I suggest to set it ULONG_MAX too. But this is really corner
case.

> 
>-	pgdat->first_deferred_pfn = spfn;
> 	pgdat_resize_unlock(pgdat, &flags);
> 
> 	return nr_pages > 0;
>-- 
>2.50.1
>

-- 
Wei Yang
Help you, Help me