linux-kernel - [PATCH v3 2/5] Xarray: move forward index correctly in xas

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Message-Id: <20241213122523.12764-3-shikemeng@huaweicloud.com>
Date: Fri, 13 Dec 2024 20:25:20 +0800
From: Kemeng Shi <shikemeng@...weicloud.com>
To: akpm@...ux-foundation.org,
	willy@...radead.org
Cc: linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org
Subject: [PATCH v3 2/5] Xarray: move forward index correctly in xas_pause()

After xas_load(), xas->index could point to mid of found multi-index entry
and xas->index's bits under node->shift maybe non-zero. The afterward
xas_pause() will move forward xas->index with xa->node->shift with bits
under node->shift un-masked and thus skip some index unexpectedly.

Consider following case:
Assume XA_CHUNK_SHIFT is 4.
xa_store_range(xa, 16, 31, ...)
xa_store(xa, 32, ...)
XA_STATE(xas, xa, 17);
xas_for_each(&xas,...)
xas_load(&xas)
/* xas->index = 17, xas->xa_offset = 1, xas->xa_node->xa_shift = 4 */
xas_pause()
/* xas->index = 33, xas->xa_offset = 2, xas->xa_node->xa_shift = 4 */
As we can see, index of 32 is skipped unexpectedly.

Fix this by mask bit under node->xa_shift when move forward index in
xas_pause().

For now, this will not cause serious problems. Only minor problem
like cachestat return less number of page status could happen.

Signed-off-by: Kemeng Shi <shikemeng@...weicloud.com>
---
 lib/test_xarray.c | 35 +++++++++++++++++++++++++++++++++++
 lib/xarray.c      |  1 +
 2 files changed, 36 insertions(+)

diff --git a/lib/test_xarray.c b/lib/test_xarray.c
index d5c5cbba33ed..6932a26f4927 100644
--- a/lib/test_xarray.c
+++ b/lib/test_xarray.c
@@ -1448,6 +1448,41 @@ static noinline void check_pause(struct xarray *xa)
 	XA_BUG_ON(xa, count != order_limit);
 
 	xa_destroy(xa);
+
+	index = 0;
+	for (order = XA_CHUNK_SHIFT; order > 0; order--) {
+		XA_BUG_ON(xa, xa_store_order(xa, index, order,
+					xa_mk_index(index), GFP_KERNEL));
+		index += 1UL << order;
+	}
+
+	index = 0;
+	count = 0;
+	xas_set(&xas, 0);
+	rcu_read_lock();
+	xas_for_each(&xas, entry, ULONG_MAX) {
+		XA_BUG_ON(xa, entry != xa_mk_index(index));
+		index += 1UL << (XA_CHUNK_SHIFT - count);
+		count++;
+	}
+	rcu_read_unlock();
+	XA_BUG_ON(xa, count != XA_CHUNK_SHIFT);
+
+	index = 0;
+	count = 0;
+	xas_set(&xas, XA_CHUNK_SIZE / 2 + 1);
+	rcu_read_lock();
+	xas_for_each(&xas, entry, ULONG_MAX) {
+		XA_BUG_ON(xa, entry != xa_mk_index(index));
+		index += 1UL << (XA_CHUNK_SHIFT - count);
+		count++;
+		xas_pause(&xas);
+	}
+	rcu_read_unlock();
+	XA_BUG_ON(xa, count != XA_CHUNK_SHIFT);
+
+	xa_destroy(xa);
+
 }
 
 static noinline void check_move_tiny(struct xarray *xa)
diff --git a/lib/xarray.c b/lib/xarray.c
index fa87949719a0..d0732c5b8403 100644
--- a/lib/xarray.c
+++ b/lib/xarray.c
@@ -1147,6 +1147,7 @@ void xas_pause(struct xa_state *xas)
 			if (!xa_is_sibling(xa_entry(xas->xa, node, offset)))
 				break;
 		}
+		xas->xa_index &= ~0UL << node->shift;
 		xas->xa_index += (offset - xas->xa_offset) << node->shift;
 		if (xas->xa_index == 0)
 			xas->xa_node = XAS_BOUNDS;
-- 
2.30.0