linux-kernel - Re: [PATCH v5 3/5] writeback: fix build problems of "writeback: support retrieving per group debug writeback stats of bdi"

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Date: Wed, 24 Apr 2024 09:27:39 -0400
From: Johannes Weiner <hannes@...xchg.org>
To: Kemeng Shi <shikemeng@...weicloud.com>
Cc: akpm@...ux-foundation.org, willy@...radead.org, jack@...e.cz,
	bfoster@...hat.com, tj@...nel.org, dsterba@...e.com,
	mjguzik@...il.com, dhowells@...hat.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v5 3/5] writeback: fix build problems of "writeback:
 support retrieving per group debug writeback stats of bdi"

Hi Kemeng,

On Tue, Apr 23, 2024 at 11:46:41AM +0800, Kemeng Shi wrote:
> Fix two build problems:
> 1. implicit declaration of function 'cgroup_ino'.

I just ran into this as well, with defconfig on mm-everything:

/home/hannes/src/linux/linux/mm/backing-dev.c: In function 'wb_stats_show':
/home/hannes/src/linux/linux/mm/backing-dev.c:175:33: error: 'struct bdi_writeback' has no member named 'memcg_css'
  175 |                    cgroup_ino(wb->memcg_css->cgroup),
      |                                 ^~
make[3]: *** [/home/hannes/src/linux/linux/scripts/Makefile.build:244: mm/backing-dev.o] Error 1

> ---
>  mm/backing-dev.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/backing-dev.c b/mm/backing-dev.c
> index 6ecd11bdce6e..e61bbb1bd622 100644
> --- a/mm/backing-dev.c
> +++ b/mm/backing-dev.c
> @@ -172,7 +172,11 @@ static void wb_stats_show(struct seq_file *m, struct bdi_writeback *wb,
>  		   "b_more_io:         %10lu\n"
>  		   "b_dirty_time:      %10lu\n"
>  		   "state:             %10lx\n\n",
> +#ifdef CONFIG_CGROUP_WRITEBACK
>  		   cgroup_ino(wb->memcg_css->cgroup),
> +#else
> +		   1ul,
> +#endif
>  		   K(stats->nr_writeback),
>  		   K(stats->nr_reclaimable),
>  		   K(stats->wb_thresh),
> @@ -192,7 +196,6 @@ static int cgwb_debug_stats_show(struct seq_file *m, void *v)
>  	unsigned long background_thresh;
>  	unsigned long dirty_thresh;
>  	struct bdi_writeback *wb;
> -	struct wb_stats stats;
>  
>  	global_dirty_limits(&background_thresh, &dirty_thresh);

The fix looks right to me, but it needs to be folded into the previous
patch. No patch should knowingly introduce an issue that is fixed
later on. This will break bisection.