Re: [PATCH v4 2/5] mm: Create non-atomic version of SetPageReserved for init use

[Pasha Tatashin <Pavel.Tatashin@microsoft.com>]

On 9/20/18 6:27 PM, Alexander Duyck wrote:
> It doesn't make much sense to use the atomic SetPageReserved at init time
> when we are using memset to clear the memory and manipulating the page
> flags via simple "&=" and "|=" operations in __init_single_page.
> 
> This patch adds a non-atomic version __SetPageReserved that can be used
> during page init and shows about a 10% improvement in initialization times
> on the systems I have available for testing. On those systems I saw
> initialization times drop from around 35 seconds to around 32 seconds to
> initialize a 3TB block of persistent memory. I believe the main advantage
> of this is that it allows for more compiler optimization as the __set_bit
> operation can be reordered whereas the atomic version cannot.
> 
> I tried adding a bit of documentation based on commit <f1dd2cd13c4> ("mm,
> memory_hotplug: do not associate hotadded memory to zones until online").
> 
> Ideally the reserved flag should be set earlier since there is a brief
> window where the page is initialization via __init_single_page and we have
> not set the PG_Reserved flag. I'm leaving that for a future patch set as
> that will require a more significant refactor.
> 
> Acked-by: Michal Hocko <mhocko@suse.com>
> Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>

Reviewed-by: Pavel Tatashin <pavel.tatashin@microsoft.com>

> ---
> 
> v4: Added comment about __set_bit vs set_bit to the patch description
> 
>  include/linux/page-flags.h |    1 +
>  mm/page_alloc.c            |    9 +++++++--
>  2 files changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 934f91ef3f54..50ce1bddaf56 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -303,6 +303,7 @@ static inline void page_init_poison(struct page *page, size_t size)
>  
>  PAGEFLAG(Reserved, reserved, PF_NO_COMPOUND)
>  	__CLEARPAGEFLAG(Reserved, reserved, PF_NO_COMPOUND)
> +	__SETPAGEFLAG(Reserved, reserved, PF_NO_COMPOUND)
>  PAGEFLAG(SwapBacked, swapbacked, PF_NO_TAIL)
>  	__CLEARPAGEFLAG(SwapBacked, swapbacked, PF_NO_TAIL)
>  	__SETPAGEFLAG(SwapBacked, swapbacked, PF_NO_TAIL)
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 712cab17f86f..29bd662fffd7 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1239,7 +1239,12 @@ void __meminit reserve_bootmem_region(phys_addr_t start, phys_addr_t end)
>  			/* Avoid false-positive PageTail() */
>  			INIT_LIST_HEAD(&page->lru);
>  
> -			SetPageReserved(page);
> +			/*
> +			 * no need for atomic set_bit because the struct
> +			 * page is not visible yet so nobody should
> +			 * access it yet.
> +			 */
> +			__SetPageReserved(page);
>  		}
>  	}
>  }
> @@ -5513,7 +5518,7 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
>  		page = pfn_to_page(pfn);
>  		__init_single_page(page, pfn, zone, nid);
>  		if (context == MEMMAP_HOTPLUG)
> -			SetPageReserved(page);
> +			__SetPageReserved(page);
>  
>  		/*
>  		 * Mark the block movable so that blocks are reserved for
>