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Date:   Sun, 29 Oct 2023 20:11:32 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Daniel Gomez <da.gomez@samsung.com>
Cc:     "minchan@kernel.org" <minchan@kernel.org>,
        "senozhatsky@chromium.org" <senozhatsky@chromium.org>,
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        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "gost.dev@samsung.com" <gost.dev@samsung.com>,
        Pankaj Raghav <p.raghav@samsung.com>
Subject: Re: [RFC PATCH 01/11] XArray: add cmpxchg order test
Message-ID: <ZT68dBiJKNLXLRZA@casper.infradead.org>
References: <20230919135536.2165715-1-da.gomez@samsung.com>
 <20231028211518.3424020-1-da.gomez@samsung.com>
 <CGME20231028211538eucas1p186e33f92dbea7030f14f7f79aa1b8d54@eucas1p1.samsung.com>
 <20231028211518.3424020-2-da.gomez@samsung.com>
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On Sat, Oct 28, 2023 at 09:15:35PM +0000, Daniel Gomez wrote:
> +static noinline void check_cmpxchg_order(struct xarray *xa)
> +{
> +	void *FIVE = xa_mk_value(5);
> +	unsigned int order = IS_ENABLED(CONFIG_XARRAY_MULTI) ? 15 : 1;

... have you tried this with CONFIG_XARRAY_MULTI deselected?
I suspect it will BUG() because orders greater than 0 are not allowed.

> +	XA_BUG_ON(xa, !xa_empty(xa));
> +	XA_BUG_ON(xa, xa_store_index(xa, 5, GFP_KERNEL) != NULL);
> +	XA_BUG_ON(xa, xa_insert(xa, 5, FIVE, GFP_KERNEL) != -EBUSY);
> +	XA_BUG_ON(xa, xa_store_order(xa, 5, order, FIVE, GFP_KERNEL));
> +	XA_BUG_ON(xa, xa_get_order(xa, 5) != order);
> +	XA_BUG_ON(xa, xa_get_order(xa, xa_to_value(FIVE)) != order);
> +	old = xa_cmpxchg(xa, 5, FIVE, NULL, GFP_KERNEL);
> +	XA_BUG_ON(xa, old != FIVE);
> +	XA_BUG_ON(xa, xa_get_order(xa, 5) != 0);
> +	XA_BUG_ON(xa, xa_get_order(xa, xa_to_value(FIVE)) != 0);
> +	XA_BUG_ON(xa, xa_get_order(xa, xa_to_value(old)) != 0);
> +	XA_BUG_ON(xa, !xa_empty(xa));

I'm not sure this is a great test.  It definitely does do what you claim
it will, but for example, it's possible that we might keep that
information for other orders.  So maybe we should have another entry at
(1 << order) that keeps the node around and could theoretically keep
the order information around for the now-NULL entry?