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

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Message-ID: <34f0dd4c-9d6a-bc66-f37d-329fb1620212@kernel.dk>
Date:   Tue, 31 Jan 2023 08:15:03 -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 8:10 AM, David Hildenbrand wrote:
> On 31.01.23 16:04, Jens Axboe wrote:
>> 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.
>>
> 
> Rather only these 2. Smth like:

Ah gotcha, that makes more sense to me. Will update the patch and
send it out.

-- 
Jens Axboe