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

[Jens Axboe <axboe@kernel.dk>]

On 1/31/23 6:48?AM, David Hildenbrand wrote:
> On 31.01.23 14:41, David Howells wrote:
>> David Hildenbrand <david@redhat.com> 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;

-- 
Jens Axboe