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

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Message-ID: <028c959d-e52a-5d08-6ac6-004ecdb3e549@kernel.dk>
Date:   Tue, 31 Jan 2023 08:04:06 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     David Hildenbrand <david@...hat.com>,
        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 1/31/23 8:02?AM, David Hildenbrand wrote:
> 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.

Are all of them debug aids only? If so, yes we should just have
node_stat_* under CONFIG_DEBUG_VM.

-- 
Jens Axboe