From: Dan Williams <dan.j.williams@intel.com>
To: Jan Kara <jack@suse.cz>
Cc: Jerome Glisse <jglisse@redhat.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
linux-block@vger.kernel.org, Linux MM <linux-mm@kvack.org>,
John Hubbard <jhubbard@nvidia.com>,
Alexander Viro <viro@zeniv.linux.org.uk>,
Johannes Thumshirn <jthumshirn@suse.de>,
Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
Ming Lei <ming.lei@redhat.com>,
Dave Chinner <david@fromorbit.com>,
Jason Gunthorpe <jgg@ziepe.ca>,
Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH v1 10/15] block: add gup flag to bio_add_page()/bio_add_pc_page()/__bio_add_page()
Date: Tue, 16 Apr 2019 09:54:21 -0700 [thread overview]
Message-ID: <CAPcyv4iKVEty4MHt+fi3Mt5qp526vVOD1Xji=wXep_KLTZ0E8A@mail.gmail.com> (raw)
In-Reply-To: <20190416164658.GB17148@quack2.suse.cz>
On Tue, Apr 16, 2019 at 9:47 AM Jan Kara <jack@suse.cz> wrote:
>
> On Mon 15-04-19 11:24:33, Jerome Glisse wrote:
> > On Mon, Apr 15, 2019 at 04:59:52PM +0200, Jan Kara wrote:
> > > Hi Jerome!
> > >
> > > On Thu 11-04-19 17:08:29, jglisse@redhat.com wrote:
> > > > From: Jérôme Glisse <jglisse@redhat.com>
> > > >
> > > > We want to keep track of how we got a reference on page added to bio_vec
> > > > ie wether the page was reference through GUP (get_user_page*) or not. So
> > > > add a flag to bio_add_page()/bio_add_pc_page()/__bio_add_page() to that
> > > > effect.
> > >
> > > Thanks for writing this patch set! Looking through patches like this one,
> > > I'm a bit concerned. With so many bio_add_page() callers it's difficult to
> > > get things right and not regress in the future. I'm wondering whether the
> > > things won't be less error-prone if we required that all page reference
> > > from bio are gup-like (not necessarily taken by GUP, if creator of the bio
> > > gets to struct page he needs via some other means (e.g. page cache lookup),
> > > he could just use get_gup_pin() helper we'd provide). After all, a page
> > > reference in bio means that the page is pinned for the duration of IO and
> > > can be DMAed to/from so it even makes some sense to track the reference
> > > like that. Then bio_put() would just unconditionally do put_user_page() and
> > > we won't have to propagate the information in the bio.
> > >
> > > Do you think this would be workable and easier?
> >
> > It might be workable but i am not sure it is any simpler. bio_add_page*()
> > does not take page reference it is up to the caller to take the proper
> > page reference so the complexity would be push there (just in a different
> > place) so i don't think it would be any simpler. This means that we would
> > have to update more code than this patchset does.
>
> I agree that the amount of work in this patch set is about the same
> (although you don't have to pass the information about reference type in
> the biovec so you save the complexities there). But for the future the
> rule that "bio references must be gup-pins" is IMO easier to grasp for
> developers and you can reasonably assert it in bio_add_page().
>
> > This present patch is just a coccinelle semantic patch and even if it
> > is scary to see that many call site, they are not that many that need
> > to worry about the GUP parameter and they all are in patch 11, 12, 13
> > and 14.
> >
> > So i believe this patchset is simpler than converting everyone to take
> > a GUP like page reference. Also doing so means we loose the information
> > about GUP kind of defeat the purpose. So i believe it would be better
> > to limit special reference to GUP only pages.
>
> So what's the difference whether the page reference has been acquired via
> GUP or via some other means? I don't think that really matters. If say
> infiniband introduced new ioctl() that takes file descriptor, offset, and
> length and just takes pages from page cache and attaches them to their RDMA
> scatter-gather lists, then they'd need to use 'pin' references anyway...
>
> Then why do we work on differentiating between GUP pins and other page
> references? Because it matters what the reference is going to be used for
> and what is it's lifetime. And generally GUP references are used to do IO
> to/from page and may even be controlled by userspace so that's why we need
> to make them different. But in principle the 'gup-pin' reference is not about
> the fact that the reference has been obtained from GUP but about the fact
> that it is used to do IO. Hence I think that the rule "bio references must
> be gup-pins" makes some sense.
+1 to this idea. I don't see the need to preserve the concept that
some biovecs carry non-GUP pages.
next prev parent reply other threads:[~2019-04-16 16:54 UTC|newest]
Thread overview: 47+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-11 21:08 [PATCH v1 00/15] Keep track of GUPed pages in fs and block jglisse
2019-04-11 21:08 ` [PATCH v1 01/15] fs/direct-io: fix trailing whitespace issues jglisse
2019-04-11 21:08 ` [PATCH v1 02/15] iov_iter: add helper to test if an iter would use GUP jglisse
2019-04-11 21:08 ` [PATCH v1 03/15] block: introduce bvec_page()/bvec_set_page() to get/set bio_vec.bv_page jglisse
2019-04-11 21:08 ` [PATCH v1 04/15] block: introduce BIO_VEC_INIT() macro to initialize bio_vec structure jglisse
2019-04-11 21:08 ` [PATCH v1 05/15] block: replace all bio_vec->bv_page by bvec_page()/bvec_set_page() jglisse
2019-04-11 21:08 ` [PATCH v1 06/15] block: convert bio_vec.bv_page to bv_pfn to store pfn and not page jglisse
2019-04-11 21:08 ` [PATCH v1 07/15] block: add bvec_put_page_dirty*() to replace put_page(bvec_page()) jglisse
2019-04-11 21:08 ` [PATCH v1 08/15] block: use bvec_put_page() instead of put_page(bvec_page()) jglisse
2019-04-11 21:08 ` [PATCH v1 09/15] block: bvec_put_page_dirty* instead of set_page_dirty* and bvec_put_page jglisse
2019-04-11 21:08 ` [PATCH v1 10/15] block: add gup flag to bio_add_page()/bio_add_pc_page()/__bio_add_page() jglisse
2019-04-15 14:59 ` Jan Kara
2019-04-15 15:24 ` Jerome Glisse
2019-04-16 16:46 ` Jan Kara
2019-04-16 16:54 ` Dan Williams [this message]
2019-04-16 17:07 ` Jerome Glisse
2019-04-16 0:22 ` Jerome Glisse
2019-04-16 16:52 ` Jan Kara
2019-04-16 18:32 ` Jerome Glisse
2019-04-11 21:08 ` [PATCH v1 11/15] block: make sure bio_add_page*() knows page that are coming from GUP jglisse
2019-04-11 21:08 ` [PATCH v1 12/15] fs/direct-io: keep track of wether a page is coming from GUP or not jglisse
2019-04-11 23:14 ` Dave Chinner
2019-04-12 0:08 ` Jerome Glisse
2019-04-11 21:08 ` [PATCH v1 13/15] fs/splice: use put_user_page() when appropriate jglisse
2019-04-11 21:08 ` [PATCH v1 14/15] fs: use bvec_set_gup_page() where appropriate jglisse
2019-04-11 21:08 ` [PATCH v1 15/15] ceph: use put_user_pages() instead of ceph_put_page_vector() jglisse
2019-04-15 7:46 ` Yan, Zheng
2019-04-15 15:11 ` Jerome Glisse
2019-04-16 0:00 ` [PATCH v1 00/15] Keep track of GUPed pages in fs and block Dave Chinner
[not found] ` <2c124cc4-b97e-ee28-2926-305bc6bc74bd@plexistor.com>
2019-04-16 18:47 ` Jerome Glisse
2019-04-16 18:59 ` Kent Overstreet
2019-04-16 19:12 ` Dan Williams
2019-04-16 19:49 ` Jerome Glisse
2019-04-17 21:53 ` Dan Williams
2019-04-17 22:28 ` Jerome Glisse
2019-04-17 23:32 ` Dan Williams
2019-04-18 10:42 ` Jan Kara
2019-04-18 14:27 ` Jerome Glisse
2019-04-18 15:30 ` Jan Kara
2019-04-18 15:36 ` Jerome Glisse
2019-04-18 18:03 ` Dan Williams
[not found] ` <ccac6c5a-7120-0455-88de-ca321b01e825@plexistor.com>
2019-04-16 19:57 ` Jerome Glisse
[not found] ` <41e2d7e1-104b-a006-2824-015ca8c76cc8@gmail.com>
2019-04-16 23:16 ` Jerome Glisse
[not found] ` <fa00a2ff-3664-3165-7af8-9d9c53238245@plexistor.com>
2019-04-17 2:03 ` Jerome Glisse
2019-04-17 21:19 ` Jerome Glisse
2019-04-16 23:34 ` Jerome Glisse
2019-04-17 21:54 ` Dan Williams
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