Commit c9ba5249 authored by Kemeng Shi's avatar Kemeng Shi Committed by Andrew Morton
Browse files

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.

Link: https://lkml.kernel.org/r/20241213122523.12764-3-shikemeng@huaweicloud.com


Signed-off-by: default avatarKemeng Shi <shikemeng@huaweicloud.com>
Cc: Mattew Wilcox <willy@infradead.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 7e060df0

lib/test_xarray.c

+35 −0
Original line number Diff line number Diff line
@@ -1511,6 +1511,41 @@ static noinline void check_pause(struct kunit *test) 
	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 kunit *test)

lib/xarray.c

+1 −0
Original line number Diff line number Diff line
@@ -1152,6 +1152,7 @@ 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;