Re: [PATCH v4] mm/page_owner.c: add llseek for page_owner

[Kassey Li]

On 8/8/2022 11:33 PM, Vlastimil Babka (SUSE) wrote:
> 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@xxxxxxxxxx>
>
> Thanks, but please change it to Vlastimil Babka <vbabka@xxxxxxx>
> It was some mistake between me and thunderbird that it was sent from @k.o.
  Ok, will change it.
> > Signed-off-by: Kassey Li <quic_yingangl@xxxxxxxxxxx>
> > ---
> >   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() ?  
  agree.
> Also we should update Documentation/mm/page_owner.rst to document the lseek
> support and that it works with a pfn.
  will do this in v5 patch set.
> 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)