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Date:   Fri, 29 Sep 2023 13:02:52 +0300
From:   Mike Rapoport <rppt@kernel.org>
To:     Yajun Deng <yajun.deng@linux.dev>
Cc:     akpm@linux-foundation.org, mike.kravetz@oracle.com,
        muchun.song@linux.dev, willy@infradead.org, david@redhat.com,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 2/2] mm: Init page count in reserve_bootmem_region
 when MEMINIT_EARLY
Message-ID: <20230929100252.GW3303@kernel.org>
References: <20230928083302.386202-1-yajun.deng@linux.dev>
 <20230928083302.386202-3-yajun.deng@linux.dev>
 <20230929083018.GU3303@kernel.org>
 <a6a20ff9-385c-639f-75cf-ce73a01d97cf@linux.dev>
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On Fri, Sep 29, 2023 at 05:50:40PM +0800, Yajun Deng wrote:
> 
> On 2023/9/29 16:30, Mike Rapoport wrote:
> > On Thu, Sep 28, 2023 at 04:33:02PM +0800, Yajun Deng wrote:
> > > memmap_init_range() would init page count of all pages, but the free
> > > pages count would be reset in __free_pages_core(). There are opposite
> > > operations. It's unnecessary and time-consuming when it's MEMINIT_EARLY
> > > context.
> > > 
> > > Init page count in reserve_bootmem_region when in MEMINIT_EARLY context,
> > > and check the page count before reset it.
> > > 
> > > At the same time, the INIT_LIST_HEAD in reserve_bootmem_region isn't
> > > need, as it already done in __init_single_page.
> > > 
> > > The following data was tested on an x86 machine with 190GB of RAM.
> > > 
> > > before:
> > > free_low_memory_core_early()    341ms
> > > 
> > > after:
> > > free_low_memory_core_early()    285ms
> > > 
> > > Signed-off-by: Yajun Deng <yajun.deng@linux.dev>
> > > ---
> > > v4: same with v2.
> > > v3: same with v2.
> > > v2: check page count instead of check context before reset it.
> > > v1: https://lore.kernel.org/all/20230922070923.355656-1-yajun.deng@linux.dev/
> > > ---
> > >   mm/mm_init.c    | 18 +++++++++++++-----
> > >   mm/page_alloc.c | 20 ++++++++++++--------
> > >   2 files changed, 25 insertions(+), 13 deletions(-)
> > > 
> > > diff --git a/mm/mm_init.c b/mm/mm_init.c
> > > index 9716c8a7ade9..3ab8861e1ef3 100644
> > > --- a/mm/mm_init.c
> > > +++ b/mm/mm_init.c
> > > @@ -718,7 +718,7 @@ static void __meminit init_reserved_page(unsigned long pfn, int nid)
> > >   		if (zone_spans_pfn(zone, pfn))
> > >   			break;
> > >   	}
> > > -	__init_single_page(pfn_to_page(pfn), pfn, zid, nid, INIT_PAGE_COUNT);
> > > +	__init_single_page(pfn_to_page(pfn), pfn, zid, nid, 0);
> > >   }
> > >   #else
> > >   static inline void pgdat_set_deferred_range(pg_data_t *pgdat) {}
> > > @@ -756,8 +756,8 @@ void __meminit reserve_bootmem_region(phys_addr_t start,
> > >   			init_reserved_page(start_pfn, nid);
> > > -			/* Avoid false-positive PageTail() */
> > > -			INIT_LIST_HEAD(&page->lru);
> > > +			/* Init page count for reserved region */
> > Please add a comment that describes _why_ we initialize the page count here.
> Okay.
> > 
> > > +			init_page_count(page);
> > >   			/*
> > >   			 * no need for atomic set_bit because the struct
> > > @@ -888,9 +888,17 @@ void __meminit memmap_init_range(unsigned long size, int nid, unsigned long zone
> > >   		}
> > >   		page = pfn_to_page(pfn);
> > > -		__init_single_page(page, pfn, zone, nid, INIT_PAGE_COUNT);
> > > -		if (context == MEMINIT_HOTPLUG)
> > > +
> > > +		/* If the context is MEMINIT_EARLY, we will init page count and
> > > +		 * mark page reserved in reserve_bootmem_region, the free region
> > > +		 * wouldn't have page count and we will check the pages count
> > > +		 * in __free_pages_core.
> > > +		 */
> > > +		__init_single_page(page, pfn, zone, nid, 0);
> > > +		if (context == MEMINIT_HOTPLUG) {
> > > +			init_page_count(page);
> > >   			__SetPageReserved(page);
> > Rather than calling init_page_count() and __SetPageReserved() for
> > MEMINIT_HOTPLUG you can set flags to INIT_PAGE_COUNT | INIT_PAGE_RESERVED
> > an call __init_single_page() after the check for MEMINIT_HOTPLUG.
> 
> No, the following code would cost more time than the current code in
> memmap_init().
> 
> if (context == MEMINIT_HOTPLUG)
> 
> 	__init_single_page(page, pfn, zone, nid, INIT_PAGE_COUNT | INIT_PAGE_RESERVED);
> else
> 	
> 	__init_single_page(page, pfn, zone, nid, 0);

Sorry if I wasn't clear. What I meant was to have something along these lines:

	enum page_init_flags flags = 0;

	if (context == MEMINIT_HOTPLUG)
		flags = INIT_PAGE_COUNT | INIT_PAGE_RESERVED;
	__init_single_page(page, pfn, zone, nid, flags);

> > But more generally, I wonder if we have to differentiate HOTPLUG here at all.
> > @David, can you comment please?
> > 
> > > +		}
> > >   		/*
> > >   		 * Usually, we want to mark the pageblock MIGRATE_MOVABLE,
> > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > index 06be8821d833..b868caabe8dc 100644
> > > --- a/mm/page_alloc.c
> > > +++ b/mm/page_alloc.c
> > > @@ -1285,18 +1285,22 @@ void __free_pages_core(struct page *page, unsigned int order)
> > >   	unsigned int loop;
> > >   	/*
> > > -	 * When initializing the memmap, __init_single_page() sets the refcount
> > > -	 * of all pages to 1 ("allocated"/"not free"). We have to set the
> > > -	 * refcount of all involved pages to 0.
> > > +	 * When initializing the memmap, memmap_init_range sets the refcount
> > > +	 * of all pages to 1 ("reserved" and "free") in hotplug context. We
> > > +	 * have to set the refcount of all involved pages to 0. Otherwise,
> > > +	 * we don't do it, as reserve_bootmem_region only set the refcount on
> > > +	 * reserve region ("reserved") in early context.
> > >   	 */
> > Again, why hotplug and early init should be different?
> I will add a comment that describes it will save boot time.

But why do we need initialize struct pages differently at boot time vs
memory hotplug?
Is there a reason memory hotplug cannot have page count set to 0 just like
for pages reserved at boot time?
 
> > > -	prefetchw(p);
> > > -	for (loop = 0; loop < (nr_pages - 1); loop++, p++) {
> > > -		prefetchw(p + 1);
> > > +	if (page_count(page)) {
> > > +		prefetchw(p);
> > > +		for (loop = 0; loop < (nr_pages - 1); loop++, p++) {
> > > +			prefetchw(p + 1);
> > > +			__ClearPageReserved(p);
> > > +			set_page_count(p, 0);
> > > +		}
> > >   		__ClearPageReserved(p);
> > >   		set_page_count(p, 0);
> > >   	}
> > > -	__ClearPageReserved(p);
> > > -	set_page_count(p, 0);
> > >   	atomic_long_add(nr_pages, &page_zone(page)->managed_pages);
> > > -- 
> > > 2.25.1
> > > 

-- 
Sincerely yours,
Mike.