linux-kernel - [PATCH v5 3/4] mm: Create non-atomic version of SetPageReserved for init use

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Date:   Tue, 25 Sep 2018 13:20:47 -0700
From:   Alexander Duyck <alexander.h.duyck@...ux.intel.com>
To:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org, linux-nvdimm@...ts.01.org
Cc:     pavel.tatashin@...rosoft.com, mhocko@...e.com,
        dave.jiang@...el.com, dave.hansen@...el.com, jglisse@...hat.com,
        rppt@...ux.vnet.ibm.com, dan.j.williams@...el.com,
        logang@...tatee.com, mingo@...nel.org,
        kirill.shutemov@...ux.intel.com
Subject: [PATCH v5 3/4] mm: Create non-atomic version of SetPageReserved for
 init use

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@...e.com>
Reviewed-by: Pavel Tatashin <pavel.tatashin@...rosoft.com>
Signed-off-by: Alexander Duyck <alexander.h.duyck@...ux.intel.com>
---

v4: Added comment about __set_bit vs set_bit to the patch description
v5: No change

 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 511447ac02cf..926ad3083b28 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1238,7 +1238,12 @@ 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);
 		}
 	}
 }
@@ -5512,7 +5517,7 @@ 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