Re: [PATCH 4/5] bio: introduce BIO_FOLL_PIN flag

From: John Hubbard <jhubbard@nvidia.com>
To: Christoph Hellwig <hch@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Ilya Dryomov <idryomov@gmail.com>, Jens Axboe <axboe@kernel.dk>,
	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>
Subject: Re: [PATCH 4/5] bio: introduce BIO_FOLL_PIN flag
Date: Mon, 24 Aug 2020 00:36:49 -0700	[thread overview]
Message-ID: <f1a9b9b8-3e88-04d1-d08e-bed77330dbd8@nvidia.com> (raw)
In-Reply-To: <efa2519d-53b7-7b08-bf85-a5c2d725282c@nvidia.com>

On 8/22/20 11:57 PM, John Hubbard wrote:
> On 8/22/20 11:25 PM, 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?
> 
> Well, I don't *think* you can get there. However, I've only been studying
> bio/block for a fairly short time, and the scattering of get_page() and
> put_page() calls in some of the paths made me wonder if, for example,
> someone was using get_page() to acquire ITER_BVEC or ITER_KVEC via
> get_page(), and release them via bio_release_pages(). It's hard to tell.
> 
> It seems like that shouldn't be part of the design. I'm asserting that
> it isn't, with this new flag. But if you're sure that this assertion is
> unnecessary, then let's just drop this patch, of course.
> 

Also, I should have done a few more subsystem conversions, before
concluding that BIO_FOLL_PIN was a good idea. Now, as I'm working through mopping
up those other subsystems, I see that nfs/direct.c for example does not have access
to a bio instance, and so the whole thing is not really a great move, at least not
for adding to the iov_iter_pin_user_pages*() APIs.

Let's just drop this patch, after all.

thanks,
-- 
John Hubbard
NVIDIA