linux-kernel - [patch 2/9] mm: shmem: save one radix tree lookup when truncating swapped pages

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Message-Id: <1375829050-12654-3-git-send-email-hannes@cmpxchg.org>
Date:	Tue,  6 Aug 2013 18:44:03 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	linux-mm@...ck.org
Cc:	Andi Kleen <andi@...stfloor.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Thelen <gthelen@...gle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Hugh Dickins <hughd@...gle.com>, Jan Kara <jack@...e.cz>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Mel Gorman <mgorman@...e.de>,
	Minchan Kim <minchan.kim@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Rik van Riel <riel@...hat.com>,
	Michel Lespinasse <walken@...gle.com>,
	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	Roman Gushchin <klamm@...dex-team.ru>,
	Ozgun Erdogan <ozgun@...usdata.com>,
	Metin Doslu <metin@...usdata.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [patch 2/9] mm: shmem: save one radix tree lookup when truncating swapped pages

Page cache radix tree slots are usually stabilized by the page lock,
but shmem's swap cookies have no such thing.  Because the overall
truncation loop is lockless, the swap entry is currently confirmed by
a tree lookup and then deleted by another tree lookup under the same
tree lock region.

Use radix_tree_delete_item() instead, which does the verification and
deletion with only one lookup.  This also allows removing the
delete-only special case from shmem_radix_tree_replace().

Signed-off-by: Johannes Weiner <hannes@...xchg.org>
---
 mm/shmem.c | 25 ++++++++++++-------------
 1 file changed, 12 insertions(+), 13 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index c93dcd6..8510534 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -242,19 +242,17 @@ static int shmem_radix_tree_replace(struct address_space *mapping,
 			pgoff_t index, void *expected, void *replacement)
 {
 	void **pslot;
-	void *item = NULL;
+	void *item;
 
 	VM_BUG_ON(!expected);
+	VM_BUG_ON(!replacement);
 	pslot = radix_tree_lookup_slot(&mapping->page_tree, index);
-	if (pslot)
-		item = radix_tree_deref_slot_protected(pslot,
-							&mapping->tree_lock);
+	if (!pslot)
+		return -ENOENT;
+	item = radix_tree_deref_slot_protected(pslot, &mapping->tree_lock);
 	if (item != expected)
 		return -ENOENT;
-	if (replacement)
-		radix_tree_replace_slot(pslot, replacement);
-	else
-		radix_tree_delete(&mapping->page_tree, index);
+	radix_tree_replace_slot(pslot, replacement);
 	return 0;
 }
 
@@ -386,14 +384,15 @@ export:
 static int shmem_free_swap(struct address_space *mapping,
 			   pgoff_t index, void *radswap)
 {
-	int error;
+	void *old;
 
 	spin_lock_irq(&mapping->tree_lock);
-	error = shmem_radix_tree_replace(mapping, index, radswap, NULL);
+	old = radix_tree_delete_item(&mapping->page_tree, index, radswap);
 	spin_unlock_irq(&mapping->tree_lock);
-	if (!error)
-		free_swap_and_cache(radix_to_swp_entry(radswap));
-	return error;
+	if (old != radswap)
+		return -ENOENT;
+	free_swap_and_cache(radix_to_swp_entry(radswap));
+	return 0;
 }
 
 /*
-- 
1.8.3.2

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