Re: [GIT PULL] iov_iter: Improve page extraction (pin or just list)

[David Hildenbrand]

On 31.01.23 15:50, Jens Axboe wrote:
> On 1/31/23 6:48?AM, David Hildenbrand wrote:
> > On 31.01.23 14:41, David Howells wrote:
> > > David Hildenbrand <david@xxxxxxxxxx> wrote:
> > >
> > > > > > percpu counters maybe - add them up at the point of viewing?
> > > > > They are percpu, see my last email. But for every 108 changes (on
> > > > > my system), they will do two atomic_long_adds(). So not very
> > > > > useful for anything but low frequency modifications.
> > > >
> > > > Can we just treat the whole acquired/released accounting as a debug mechanism
> > > > to detect missing releases and do it only for debug kernels?
> > > >
> > > >
> > > > The pcpu counter is an s8, so we have to flush on a regular basis and cannot
> > > > really defer it any longer ... but I'm curious if it would be of any help to
> > > > only have a single PINNED counter that goes into both directions (inc/dec on
> > > > pin/release), to reduce the flushing.
> > > >
> > > > Of course, once we pin/release more than ~108 pages in one go or we switch
> > > > CPUs frequently it won't be that much of a help ...
> > >
> > > What are the stats actually used for?  Is it just debugging, or do we actually
> > > have users for them (control groups spring to mind)?
> >
> > As it's really just "how many pinning events" vs. "how many unpinning
> > events", I assume it's only for debugging.
> >
> > For example, if you pin the same page twice it would not get accounted
> > as "a single page is pinned".
>
> How about something like the below then? I can send it out as a real
> patch, will run a sanity check on it first but would be surprised if
> this doesn't fix it.
>
>
> diff --git a/mm/gup.c b/mm/gup.c
> index f45a3a5be53a..41abb16286ec 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -168,7 +168,9 @@ struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags)
>   		 */
>   		smp_mb__after_atomic();
>   
> +#ifdef CONFIG_DEBUG_VM
>   		node_stat_mod_folio(folio, NR_FOLL_PIN_ACQUIRED, refs);
> +#endif
>   
>   		return folio;
>   	}
> @@ -180,7 +182,9 @@ struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags)
>   static void gup_put_folio(struct folio *folio, int refs, unsigned int flags)
>   {
>   	if (flags & FOLL_PIN) {
> +#ifdef CONFIG_DEBUG_VM
>   		node_stat_mod_folio(folio, NR_FOLL_PIN_RELEASED, refs);
> +#endif
>   		if (folio_test_large(folio))
>   			atomic_sub(refs, folio_pincount_ptr(folio));
>   		else
> @@ -236,8 +240,9 @@ int __must_check try_grab_page(struct page *page, unsigned int flags)
>   		} else {
>   			folio_ref_add(folio, GUP_PIN_COUNTING_BIAS);
>   		}
> -
> +#ifdef CONFIG_DEBUG_VM
>   		node_stat_mod_folio(folio, NR_FOLL_PIN_ACQUIRED, 1);
> +#endif
>   	}
>   
>   	return 0;

We might want to hide the counters completely by defining them only with 
CONFIG_DEBUG_VM.

--
Thanks,

David / dhildenb