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

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Date:   Tue, 31 Jan 2023 09:54:48 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Jan Kara <jack@...e.cz>, David Hildenbrand <david@...hat.com>
Cc:     Jens Axboe <axboe@...nel.dk>, David Howells <dhowells@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        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 04:28, Jan Kara wrote:
> On Tue 31-01-23 09:32:27, David Hildenbrand wrote:
>> On 30.01.23 23:15, Jens Axboe wrote:
>>> On 1/30/23 3:12 PM, David Howells wrote:
>>>> John Hubbard <jhubbard@...dia.com> wrote:
>>>>
>>>>> This is something that we say when adding pin_user_pages_fast(),
>>>>> yes. I doubt that I can quickly find the email thread, but we
>>>>> measured it and weren't immediately able to come up with a way
>>>>> to make it faster.
>>>>
>>>> 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?
> 
> Yes, AFAIK it is just a debug mechanism for helping to find out issues with
> page pinning conversions. So I think we can put this behind some debugging
> ifdef. John?
> 

Yes, just for debugging. I wrote a little note just now in response to
the patch about how we ended up here: "yes, it's time to hide these
behind an ifdef".

thanks,
-- 
John Hubbard
NVIDIA