linux-kernel - [PATCH 17/18] mm, hugetlb: retry if we fail to allocate a hugepage with use

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Message-Id: <1375075929-6119-18-git-send-email-iamjoonsoo.kim@lge.com>
Date:	Mon, 29 Jul 2013 14:32:08 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Michal Hocko <mhocko@...e.cz>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Hugh Dickins <hughd@...gle.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	David Gibson <david@...son.dropbear.id.au>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Joonsoo Kim <js1304@...il.com>,
	Wanpeng Li <liwanp@...ux.vnet.ibm.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Hillf Danton <dhillf@...il.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: [PATCH 17/18] mm, hugetlb: retry if we fail to allocate a hugepage with use_reserve

If parallel fault occur, we can fail to allocate a hugepage,
because many threads dequeue a hugepage to handle a fault of same address.
This makes reserved pool shortage just for a little while and this cause
faulting thread who is ensured to have enough reserved hugepages
to get a SIGBUS signal.

To solve this problem, we already have a nice solution, that is,
a hugetlb_instantiation_mutex. This blocks other threads to dive into
a fault handler. This solve the problem clearly, but it introduce
performance degradation, because it serialize all fault handling.

Now, I try to remove a hugetlb_instantiation_mutex to get rid of
performance degradation. A prerequisite is that other thread should
not get a SIGBUS if they are ensured to have enough reserved pages.

For this purpose, if we fail to allocate a new hugepage with use_reserve,
we return just 0, instead of VM_FAULT_SIGBUS. use_reserve
represent that this user is legimate one who are ensured to have enough
reserved pages. This prevent these thread not to get a SIGBUS signal and
make these thread retrying fault handling.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 6a9ec69..909075b 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2623,7 +2623,10 @@ retry_avoidcopy:
 			WARN_ON_ONCE(1);
 		}

-		ret = VM_FAULT_SIGBUS;
+		if (use_reserve)
+			ret = 0;
+		else
+			ret = VM_FAULT_SIGBUS;
 		goto out_lock;
 	}

@@ -2741,7 +2744,10 @@ retry:

 		page = alloc_huge_page(vma, address, use_reserve);
 		if (IS_ERR(page)) {
-			ret = VM_FAULT_SIGBUS;
+			if (use_reserve)
+				ret = 0;
+			else
+				ret = VM_FAULT_SIGBUS;
 			goto out;
 		}
 		clear_huge_page(page, address, pages_per_huge_page(h));
-- 
1.7.9.5

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