Message-ID: <f2fb6cc5-ff95-ca51-b377-5e4bd239d5e8@kernel.dk>
Date:   Tue, 31 Jan 2023 07:50:45 -0700
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Subject: Re: [GIT PULL] iov_iter: Improve page extraction (pin or just list)
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To:     David Hildenbrand <david@redhat.com>,
        David Howells <dhowells@redhat.com>
Cc:     John Hubbard <jhubbard@nvidia.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@infradead.org>,
        Matthew Wilcox <willy@infradead.org>, Jan Kara <jack@suse.cz>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Logan Gunthorpe <logang@deltatee.com>,
        Jeff Layton <jlayton@kernel.org>, linux-block@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <040ed7a7-3f4d-dab7-5a49-1cd9933c5445@redhat.com>
 <e68c5cab-c3a6-1872-98fa-9f909f23be79@nvidia.com>
 <3351099.1675077249@warthog.procyon.org.uk>
 <fd0003a0-a133-3daf-891c-ba7deafad768@kernel.dk>
 <f57ee72f-38e9-6afa-182f-2794638eadcb@kernel.dk>
 <e8480b18-08af-d101-a721-50d213893492@kernel.dk>
 <3520518.1675116740@warthog.procyon.org.uk>
 <f392399b-a4c4-2251-e12b-e89fff351c4d@kernel.dk>
 <3791872.1675172490@warthog.procyon.org.uk>
 <88d50843-9aa6-7930-433d-9b488857dc14@redhat.com>
From:   Jens Axboe <axboe@kernel.dk>
In-Reply-To: <88d50843-9aa6-7930-433d-9b488857dc14@redhat.com>
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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