Message-ID: <7f8f2d0f-4bf2-71aa-c356-c78c6b7fd071@redhat.com>
Date:   Tue, 31 Jan 2023 16:02:21 +0100
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Subject: Re: [GIT PULL] iov_iter: Improve page extraction (pin or just list)
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To:     Jens Axboe <axboe@kernel.dk>, 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>
 <f2fb6cc5-ff95-ca51-b377-5e4bd239d5e8@kernel.dk>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <f2fb6cc5-ff95-ca51-b377-5e4bd239d5e8@kernel.dk>
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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@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;
> 

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

-- 
Thanks,

David / dhildenb