linux-kernel - Re: [PATCH v2 1/2] printk_ringbuffer: don't needlessly wrap data blocks around

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Message-ID: <0ffb6fad-20cb-4e87-bfa6-a2e6124c03ad@yandex-team.ru>
Date: Fri, 26 Sep 2025 17:35:53 +0300
From: Daniil Tatianin <d-tatianin@...dex-team.ru>
To: Petr Mladek <pmladek@...e.com>
Cc: linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
 John Ogness <john.ogness@...utronix.de>,
 Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: Re: [PATCH v2 1/2] printk_ringbuffer: don't needlessly wrap data
 blocks around

Hi all!

Now that the fix for large messages is committed, I think this patch 
should be good to take,
and the second one we can just drop since it's no longer relevant.

Thanks,
Daniil

On 9/5/25 5:41 PM, Daniil Tatianin wrote:
> Previously, data blocks that perfectly fit the data ring buffer would
> get wrapped around to the beginning for no reason since the calculated
> offset of the next data block would belong to the next wrap. Since this
> offset is not actually part of the data block, but rather the offset of
> where the next data block is going to start, there is no reason to
> include it when deciding whether the current block fits the buffer.
>
> Signed-off-by: Daniil Tatianin <d-tatianin@...dex-team.ru>
> ---
>   kernel/printk/printk_ringbuffer.c | 27 +++++++++++++++++++--------
>   1 file changed, 19 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c
> index d9fb053cff67..99989a9ce4b4 100644
> --- a/kernel/printk/printk_ringbuffer.c
> +++ b/kernel/printk/printk_ringbuffer.c
> @@ -1002,6 +1002,17 @@ static bool desc_reserve(struct printk_ringbuffer *rb, unsigned long *id_out)
>   	return true;
>   }
>   
> +static bool is_blk_wrapped(struct prb_data_ring *data_ring,
> +			    unsigned long begin_lpos, unsigned long next_lpos)
> +{
> +	/*
> +	 * Subtract one from next_lpos since it's not actually part of this data
> +	 * block. This allows perfectly fitting records to not wrap.
> +	 */
> +	return DATA_WRAPS(data_ring, begin_lpos) !=
> +	       DATA_WRAPS(data_ring, next_lpos - 1);
> +}
> +
>   /* Determine the end of a data block. */
>   static unsigned long get_next_lpos(struct prb_data_ring *data_ring,
>   				   unsigned long lpos, unsigned int size)
> @@ -1013,7 +1024,7 @@ static unsigned long get_next_lpos(struct prb_data_ring *data_ring,
>   	next_lpos = lpos + size;
>   
>   	/* First check if the data block does not wrap. */
> -	if (DATA_WRAPS(data_ring, begin_lpos) == DATA_WRAPS(data_ring, next_lpos))
> +	if (!is_blk_wrapped(data_ring, begin_lpos, next_lpos))
>   		return next_lpos;
>   
>   	/* Wrapping data blocks store their data at the beginning. */
> @@ -1081,7 +1092,7 @@ static char *data_alloc(struct printk_ringbuffer *rb, unsigned int size,
>   	blk = to_block(data_ring, begin_lpos);
>   	blk->id = id; /* LMM(data_alloc:B) */
>   
> -	if (DATA_WRAPS(data_ring, begin_lpos) != DATA_WRAPS(data_ring, next_lpos)) {
> +	if (is_blk_wrapped(data_ring, begin_lpos, next_lpos)) {
>   		/* Wrapping data blocks store their data at the beginning. */
>   		blk = to_block(data_ring, 0);
>   
> @@ -1125,7 +1136,7 @@ static char *data_realloc(struct printk_ringbuffer *rb, unsigned int size,
>   		return NULL;
>   
>   	/* Keep track if @blk_lpos was a wrapping data block. */
> -	wrapped = (DATA_WRAPS(data_ring, blk_lpos->begin) != DATA_WRAPS(data_ring, blk_lpos->next));
> +	wrapped = is_blk_wrapped(data_ring, blk_lpos->begin, blk_lpos->next);
>   
>   	size = to_blk_size(size);
>   
> @@ -1151,7 +1162,7 @@ static char *data_realloc(struct printk_ringbuffer *rb, unsigned int size,
>   
>   	blk = to_block(data_ring, blk_lpos->begin);
>   
> -	if (DATA_WRAPS(data_ring, blk_lpos->begin) != DATA_WRAPS(data_ring, next_lpos)) {
> +	if (is_blk_wrapped(data_ring, blk_lpos->begin, next_lpos)) {
>   		struct prb_data_block *old_blk = blk;
>   
>   		/* Wrapping data blocks store their data at the beginning. */
> @@ -1187,7 +1198,7 @@ static unsigned int space_used(struct prb_data_ring *data_ring,
>   	if (BLK_DATALESS(blk_lpos))
>   		return 0;
>   
> -	if (DATA_WRAPS(data_ring, blk_lpos->begin) == DATA_WRAPS(data_ring, blk_lpos->next)) {
> +	if (!is_blk_wrapped(data_ring, blk_lpos->begin, blk_lpos->next)) {
>   		/* Data block does not wrap. */
>   		return (DATA_INDEX(data_ring, blk_lpos->next) -
>   			DATA_INDEX(data_ring, blk_lpos->begin));
> @@ -1234,14 +1245,14 @@ static const char *get_data(struct prb_data_ring *data_ring,
>   	}
>   
>   	/* Regular data block: @begin less than @next and in same wrap. */
> -	if (DATA_WRAPS(data_ring, blk_lpos->begin) == DATA_WRAPS(data_ring, blk_lpos->next) &&
> +	if (!is_blk_wrapped(data_ring, blk_lpos->begin, blk_lpos->next) &&
>   	    blk_lpos->begin < blk_lpos->next) {
>   		db = to_block(data_ring, blk_lpos->begin);
>   		*data_size = blk_lpos->next - blk_lpos->begin;
>   
>   	/* Wrapping data block: @begin is one wrap behind @next. */
> -	} else if (DATA_WRAPS(data_ring, blk_lpos->begin + DATA_SIZE(data_ring)) ==
> -		   DATA_WRAPS(data_ring, blk_lpos->next)) {
> +	} else if (!is_blk_wrapped(data_ring,
> +		   blk_lpos->begin + DATA_SIZE(data_ring), blk_lpos->next)) {
>   		db = to_block(data_ring, 0);
>   		*data_size = DATA_INDEX(data_ring, blk_lpos->next);
>