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

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Date:   Tue, 31 Jan 2023 16:02:21 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Jens Axboe <axboe@...nel.dk>, David Howells <dhowells@...hat.com>
Cc:     John Hubbard <jhubbard@...dia.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Jason Gunthorpe <jgg@...dia.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Jeff Layton <jlayton@...nel.org>, linux-block@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [GIT PULL] iov_iter: Improve page extraction (pin or just list)

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@...hat.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