Message-ID: <0fc7fded-f6f5-e7e4-1b3d-3dfd224d9458@nvidia.com>
Date:   Tue, 31 Jan 2023 09:54:48 -0800
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Subject: Re: [GIT PULL] iov_iter: Improve page extraction (pin or just list)
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To:     Jan Kara <jack@suse.cz>, David Hildenbrand <david@redhat.com>
Cc:     Jens Axboe <axboe@kernel.dk>, David Howells <dhowells@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@infradead.org>,
        Matthew Wilcox <willy@infradead.org>,
        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: <e68c5cab-c3a6-1872-98fa-9f909f23be79@nvidia.com>
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 <3520518.1675116740@warthog.procyon.org.uk>
 <f392399b-a4c4-2251-e12b-e89fff351c4d@kernel.dk>
 <040ed7a7-3f4d-dab7-5a49-1cd9933c5445@redhat.com>
 <20230131122857.6tchetnsgepl3ck3@quack3>
From:   John Hubbard <jhubbard@nvidia.com>
In-Reply-To: <20230131122857.6tchetnsgepl3ck3@quack3>
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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@nvidia.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