Date:   Tue, 31 Jan 2023 13:28:57 +0100
From:   Jan Kara <jack@suse.cz>
To:     David Hildenbrand <david@redhat.com>
Cc:     Jens Axboe <axboe@kernel.dk>, David Howells <dhowells@redhat.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@infradead.org>,
        Matthew Wilcox <willy@infradead.org>, Jan Kara <jack@suse.cz>,
        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
Subject: Re: [GIT PULL] iov_iter: Improve page extraction (pin or just list)
Message-ID: <20230131122857.6tchetnsgepl3ck3@quack3>
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>
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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?

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR