Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [PATCH 4/5] bio: introduce BIO_FOLL_PIN flag
To:     Christoph Hellwig <hch@infradead.org>,
        John Hubbard <jhubbard@nvidia.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Ilya Dryomov <idryomov@gmail.com>,
        Jeff Layton <jlayton@kernel.org>, linux-xfs@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
        ceph-devel@vger.kernel.org, linux-mm@kvack.org,
        LKML <linux-kernel@vger.kernel.org>
References: <20200822042059.1805541-1-jhubbard@nvidia.com>
 <20200822042059.1805541-5-jhubbard@nvidia.com>
 <20200823062559.GA32480@infradead.org>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <d75ce230-6c8d-8623-49a2-500835f6cdfc@kernel.dk>
Date:   Mon, 24 Aug 2020 03:20:26 -0600
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101
 Thunderbird/68.10.0
MIME-Version: 1.0
In-Reply-To: <20200823062559.GA32480@infradead.org>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 8/23/20 12:25 AM, Christoph Hellwig wrote:
> On Fri, Aug 21, 2020 at 09:20:58PM -0700, John Hubbard wrote:
>> Add a new BIO_FOLL_PIN flag to struct bio, whose "short int" flags field
>> was full, thuse triggering an expansion of the field from 16, to 32
>> bits. This allows for a nice assertion in bio_release_pages(), that the
>> bio page release mechanism matches the page acquisition mechanism.
>>
>> Set BIO_FOLL_PIN whenever pin_user_pages_fast() is used, and check for
>> BIO_FOLL_PIN before using unpin_user_page().
> 
> When would the flag not be set when BIO_NO_PAGE_REF is not set?
> 
> Also I don't think we can't just expand the flags field, but I can send
> a series to kill off two flags.

(not relevant to this series as this patch has thankfully already been
dropped, just in general - but yes, definitely need a *strong* justification
to bump the bio size).

Would actually be nice to kill off a few flags, if possible, so the
flags space isn't totally full.

-- 
Jens Axboe