linux-kernel - Re: [PATCH v4] mm/page_owner.c: add llseek for page

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Date:   Mon, 8 Aug 2022 17:33:21 +0200
From:   "Vlastimil Babka (SUSE)" <vbabka@...nel.org>
To:     Kassey Li <quic_yingangl@...cinc.com>, akpm@...ux-foundation.org
Cc:     minchan@...nel.org, iamjoonsoo.kim@....com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v4] mm/page_owner.c: add llseek for page_owner

On 8/8/22 09:36, Kassey Li wrote:
> There is usage to dump a given cma region page_owner
> instead of all page's.
> 
> This change allows to specify a ppos as start_pfn
> by fseek.
> 
> Any invalid ppos will be skipped, so it did not
> broken the origin dump feature.
> 
> Suggested-by: Vlastimil Babka (SUSE) <vbabka@...nel.org>

Thanks, but please change it to Vlastimil Babka <vbabka@...e.cz>
It was some mistake between me and thunderbird that it was sent from @k.o.

> Signed-off-by: Kassey Li <quic_yingangl@...cinc.com>
> ---
>  mm/page_owner.c | 20 ++++++++++++++++++--
>  1 file changed, 18 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page_owner.c b/mm/page_owner.c
> index e4c6f3f1695b..231b1877af99 100644
> --- a/mm/page_owner.c
> +++ b/mm/page_owner.c
> @@ -497,8 +497,8 @@ read_page_owner(struct file *file, char __user *buf, size_t count, loff_t *ppos)
>  		return -EINVAL;
>  
>  	page = NULL;
> -	pfn = min_low_pfn + *ppos;
>  
> +	pfn = *ppos;

Maybe we could still optimize the typical case a bit such that when it's 0,
we skip to min_low_pfn immediately.

>  	/* Find a valid PFN or the start of a MAX_ORDER_NR_PAGES area */
>  	while (!pfn_valid(pfn) && (pfn & (MAX_ORDER_NR_PAGES - 1)) != 0)
>  		pfn++;
> @@ -561,7 +561,7 @@ read_page_owner(struct file *file, char __user *buf, size_t count, loff_t *ppos)
>  			continue;
>  
>  		/* Record the next PFN to read in the file offset */
> -		*ppos = (pfn - min_low_pfn) + 1;
> +		*ppos = pfn + 1;
>  
>  		return print_page_owner(buf, count, pfn, page,
>  				page_owner, handle);
> @@ -570,6 +570,21 @@ read_page_owner(struct file *file, char __user *buf, size_t count, loff_t *ppos)
>  	return 0;
>  }
>  
> +static loff_t llseek_page_owner(struct file *file, loff_t offset, int whence)
> +{
> +	loff_t retval = 0;
> +	switch (whence) {
> +		case SEEK_CUR:

It's not correct for SEEK_CUR to behave like this, no?
Can we perhaps reuse the existing mem_lseek() ?

Also we should update Documentation/mm/page_owner.rst to document the lseek
support and that it works with a pfn.

Thanks!

> +		case SEEK_SET:
> +			file->f_pos = offset;
> +			break;
> +		default:
> +			retval = -ENXIO;
> +	}
> +
> +	return retval;
> +}
> +
>  static void init_pages_in_zone(pg_data_t *pgdat, struct zone *zone)
>  {
>  	unsigned long pfn = zone->zone_start_pfn;
> @@ -660,6 +675,7 @@ static void init_early_allocated_pages(void)
>  
>  static const struct file_operations proc_page_owner_operations = {
>  	.read		= read_page_owner,
> +	.llseek 	= llseek_page_owner,
>  };
>  
>  static int __init pageowner_init(void)