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

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Date:   Tue, 30 Jul 2019 11:57:02 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Christoph Hellwig <hch@...radead.org>, john.hubbard@...il.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Anna Schumaker <anna.schumaker@...app.com>,
        "David S . Miller" <davem@...emloft.net>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Eric Van Hensbergen <ericvh@...il.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jason Wang <jasowang@...hat.com>, Jens Axboe <axboe@...nel.dk>,
        Latchesar Ionkov <lucho@...kov.net>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Matthew Wilcox <willy@...radead.org>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>, ceph-devel@...r.kernel.org,
        kvm@...r.kernel.org, linux-block@...r.kernel.org,
        linux-cifs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-nfs@...r.kernel.org, linux-rdma@...r.kernel.org,
        netdev@...r.kernel.org, samba-technical@...ts.samba.org,
        v9fs-developer@...ts.sourceforge.net,
        virtualization@...ts.linux-foundation.org,
        John Hubbard <jhubbard@...dia.com>,
        Minwoo Im <minwoo.im.dev@...il.com>
Subject: Re: [PATCH 03/12] block: bio_release_pages: use flags arg instead of
 bool

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