Re: [PATCH 03/12] block: bio_release_pages: use flags arg instead of bool

From: Jerome Glisse <jglisse@redhat.com>
To: Christoph Hellwig <hch@lst.de>
Cc: kvm@vger.kernel.org, "Michael S . Tsirkin" <mst@redhat.com>,
	virtualization@lists.linux-foundation.org, linux-mm@kvack.org,
	linux-cifs@vger.kernel.org, Miklos Szeredi <miklos@szeredi.hu>,
	linux-rdma@vger.kernel.org, Matthew Wilcox <willy@infradead.org>,
	Christoph Hellwig <hch@infradead.org>,
	Jason Gunthorpe <jgg@ziepe.ca>,
	Minwoo Im <minwoo.im.dev@gmail.com>,
	v9fs-developer@lists.sourceforge.net,
	Eric Van Hensbergen <ericvh@gmail.com>,
	John Hubbard <jhubbard@nvidia.com>,
	john.hubbard@gmail.com, linux-block@vger.kernel.org,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	ceph-devel@vger.kernel.org,
	Trond Myklebust <trond.myklebust@hammerspace.com>,
	Jens Axboe <axboe@kernel.dk>,
	linux-nfs@vger.kernel.org, netdev@vger.kernel.org,
	samba-technical@lists.samba.org,
	LKML <linux-kernel@vger.kernel.org>,
	"David S . Miller" <davem@davemloft.net>,
	linux-fsdevel@vger.kernel.org, Andrew
Subject: Re: [PATCH 03/12] block: bio_release_pages: use flags arg instead of bool
Date: Tue, 30 Jul 2019 11:57:02 -0400	[thread overview]
Message-ID: <20190730155702.GB10366__5622.62215405411$1564502797$gmane$org@redhat.com> (raw)
In-Reply-To: <20190730102557.GA1700@lst.de>

On Tue, Jul 30, 2019 at 12:25:57PM +0200, Christoph Hellwig wrote:
> On Mon, Jul 29, 2019 at 04:57:21PM -0400, Jerome Glisse wrote:
> > > All pages releases by bio_release_pages should come from
> > > get_get_user_pages, so I don't really see the point here.
> > 
> > No they do not all comes from GUP for see various callers
> > of bio_check_pages_dirty() for instance iomap_dio_zero()
> > 
> > I have carefully tracked down all this and i did not do
> > anyconvertion just for the fun of it :)
> 
> Well, the point is _should_ not necessarily do.  iomap_dio_zero adds the
> ZERO_PAGE, which we by definition don't need to refcount.  So we can
> mark this bio BIO_NO_PAGE_REF safely after removing the get_page there.
> 
> Note that the equivalent in the old direct I/O code, dio_refill_pages,
> will be a little more complicated as it can match user pages and the
> ZERO_PAGE in a single bio, so a per-bio flag won't handle it easily.
> Maybe we just need to use a separate bio there as well.
> 
> In general with series like this we should not encode the status quo an
> pile new hacks upon the old one, but thing where we should be and fix
> up the old warts while having to wade through all that code.

Other user can also add page that are not coming from GUP but need to
have a reference see __blkdev_direct_IO() saddly bio get fill from many
different places and not always with GUP. So we can not say that all
pages here are coming from bio. I had a different version of the patchset
i think that was adding a new release dirty function for GUP versus non
GUP bio. I posted it a while ago, i will try to dig it up once i am
back.

Cheers,
Jérôme