linux-ext4 - Re: [RESEND PATCH v10 08/25] dept: Apply sdt_might_sleep_{start,end}() to PG

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Date:   Mon, 21 Aug 2023 05:09:44 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Byungchul Park <byungchul@...com>
Cc:     linux-kernel@...r.kernel.org, kernel_team@...ynix.com,
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Subject: Re: [RESEND PATCH v10 08/25] dept: Apply
 sdt_might_sleep_{start,end}() to PG_{locked,writeback} wait

On Mon, Aug 21, 2023 at 12:46:20PM +0900, Byungchul Park wrote:
> @@ -1219,6 +1220,9 @@ static inline bool folio_trylock_flag(struct folio *folio, int bit_nr,
>  /* How many times do we accept lock stealing from under a waiter? */
>  int sysctl_page_lock_unfairness = 5;
>  
> +static struct dept_map __maybe_unused PG_locked_map = DEPT_MAP_INITIALIZER(PG_locked_map, NULL);
> +static struct dept_map __maybe_unused PG_writeback_map = DEPT_MAP_INITIALIZER(PG_writeback_map, NULL);

Hmm, why are these "maybe unused"?  *digs*.  Ah.  Because
sdt_might_sleep_start() becomes a no-op macro if DEPT is disabled.

OK, the right way to handle this is

#ifdef CONFIG_DEPT
#define DEPT_MAP(name)	static struct dept_map name = \
		DEPT_MAP_INITIALIZER(name, NULL)
#else
#define DEPT_MAP(name)	/* */
#endif

And now DEPT takes up no space if disabled.

/* */; is a somewhat unusual thing to see, but since this must work at
top level, we can't use "do { } while (0)" like we usually do.  Given
where else this is likely to be used, i don't think it's going to be
a problem ...