From: Logan Gunthorpe <logang@deltatee.com>
To: Christoph Hellwig <hch@lst.de>
Cc: linux-kernel@vger.kernel.org, linux-block@vger.kernel.org,
linux-nvme@lists.infradead.org, linux-pci@vger.kernel.org,
linux-rdma@vger.kernel.org, Jens Axboe <axboe@kernel.dk>,
Bjorn Helgaas <bhelgaas@google.com>,
Dan Williams <dan.j.williams@intel.com>,
Sagi Grimberg <sagi@grimberg.me>, Keith Busch <kbusch@kernel.org>,
Jason Gunthorpe <jgg@ziepe.ca>,
Stephen Bates <sbates@raithlin.com>
Subject: Re: [RFC PATCH 00/28] Removing struct page from P2PDMA
Date: Mon, 24 Jun 2019 10:07:56 -0600 [thread overview]
Message-ID: <558a27ba-e7c9-9d94-cad0-377b8ee374a6@deltatee.com> (raw)
In-Reply-To: <20190624072752.GA3954@lst.de>
On 2019-06-24 1:27 a.m., Christoph Hellwig wrote:
> This is not going to fly.
>
> For one passing a dma_addr_t through the block layer is a layering
> violation, and one that I think will also bite us in practice.
> The host physical to PCIe bus address mapping can have offsets, and
> those offsets absolutely can be different for differnet root ports.
> So with your caller generated dma_addr_t everything works fine with
> a switched setup as the one you are probably testing on, but on a
> sufficiently complicated setup with multiple root ports it can break.
I don't follow this argument. Yes, I understand PCI Bus offsets and yes
I understand that they only apply beyond the bus they're working with.
But this isn't *that* complicated and it should be the responsibility of
the P2PDMA code to sort out and provide a dma_addr_t for. The dma_addr_t
that's passed through the block layer could be a bus address or it could
be the result of a dma_map_* request (if the transaction is found to go
through an RC) depending on the requirements of the devices being used.
> Also duplicating the whole block I/O stack, including hooks all over
> the fast path is pretty much a no-go.
There was very little duplicate code in the patch set. (Really just the
mapping code). There are a few hooks, but in practice not that many if
we ignore the WARN_ONs. We might be able to work to reduce this further.
The main hooks are: when we skip bouncing, when we skip integrity prep,
when we split, and when we map. And the patchset drops the PCI_P2PDMA
hook when we map. So we're talking about maybe three or four extra ifs
that would likely normally be fast due to the branch predictor.
> I've been pondering for a while if we wouldn't be better off just
> passing a phys_addr_t + len instead of the page, offset, len tuple
> in the bio_vec, though. If you look at the normal I/O path here
> is what we normally do:
>
> - we get a page as input, either because we have it at hand (e.g.
> from the page cache) or from get_user_pages (which actually caculates
> it from a pfn in the page tables)
> - once in the bio all the merging decisions are based on the physical
> address, so we have to convert it to the physical address there,
> potentially multiple times
> - then dma mapping all works off the physical address, which it gets
> from the page at the start
> - then only the dma address is used for the I/O
> - on I/O completion we often but not always need the page again. In
> the direct I/O case for reference counting and dirty status, in the
> file system also for things like marking the page uptodate
>
> So if we move to a phys_addr_t we'd need to go back to the page at least
> once. But because of how the merging works we really only need to do
> it once per segment, as we can just do pointer arithmerics do get the
> following pages. As we generally go at least once from a physical
> address to a page in the merging code even a relatively expensive vmem_map
> looks shouldn't be too bad. Even more so given that the super hot path
> (small blkdev direct I/O) can actually trivially cache the affected pages
> as well.
I've always wondered why it wasn't done this way. Passing around a page
pointer *and* an offset always seemed less efficient than just a
physical address. If we did do this, the proposed dma_addr_t and
phys_addr_t paths through the block layer could be a lot more similar as
things like the split calculation could work on either address type.
We'd just have to prevent bouncing and integrity and change have a hook
on how it's mapped.
> Linus kinda hates the pfn approach, but part of that was really that
> it was proposed for file system data, which we all found out really
> can't work as-is without pages the hard way. Another part probably
> was potential performance issue, but between the few page lookups, and
> the fact that using a single phys_addr_t instead of pfn/page + offset
> should avoid quite a few calculations performance should not actually
> be affected, although we'll have to be careful to actually verify that.
Yes, I'd agree that removing the offset should make things simpler. But
that requires changing a lot of stuff and doesn't really help what I'm
trying to do.
Logan
next prev parent reply other threads:[~2019-06-24 16:08 UTC|newest]
Thread overview: 89+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-06-20 16:12 [RFC PATCH 00/28] Removing struct page from P2PDMA Logan Gunthorpe
2019-06-20 16:12 ` [RFC PATCH 01/28] block: Introduce DMA direct request type Logan Gunthorpe
2019-06-20 16:12 ` [RFC PATCH 02/28] block: Add dma_vec structure Logan Gunthorpe
2019-06-20 16:12 ` [RFC PATCH 03/28] block: Warn on mis-use of dma-direct bios Logan Gunthorpe
2019-06-20 16:12 ` [RFC PATCH 04/28] block: Never bounce " Logan Gunthorpe
2019-06-20 17:23 ` Jason Gunthorpe
2019-06-20 18:38 ` Logan Gunthorpe
2019-06-20 16:12 ` [RFC PATCH 05/28] block: Skip dma-direct bios in bio_integrity_prep() Logan Gunthorpe
2019-06-20 16:12 ` [RFC PATCH 06/28] block: Support dma-direct bios in bio_advance_iter() Logan Gunthorpe
2019-06-20 16:12 ` [RFC PATCH 07/28] block: Use dma_vec length in bio_cur_bytes() for dma-direct bios Logan Gunthorpe
2019-06-20 16:12 ` [RFC PATCH 08/28] block: Introduce dmavec_phys_mergeable() Logan Gunthorpe
2019-06-20 16:12 ` [RFC PATCH 09/28] block: Introduce vec_gap_to_prev() Logan Gunthorpe
2019-06-20 16:12 ` [RFC PATCH 10/28] block: Create generic vec_split_segs() from bvec_split_segs() Logan Gunthorpe
2019-06-20 16:12 ` [RFC PATCH 11/28] block: Create blk_segment_split_ctx Logan Gunthorpe
2019-06-20 16:12 ` [RFC PATCH 12/28] block: Create helper for bvec_should_split() Logan Gunthorpe
2019-06-20 16:12 ` [RFC PATCH 13/28] block: Generalize bvec_should_split() Logan Gunthorpe
2019-06-20 16:12 ` [RFC PATCH 14/28] block: Support splitting dma-direct bios Logan Gunthorpe
2019-06-20 16:12 ` [RFC PATCH 15/28] block: Support counting dma-direct bio segments Logan Gunthorpe
2019-06-20 16:12 ` [RFC PATCH 16/28] block: Implement mapping dma-direct requests to SGs in blk_rq_map_sg() Logan Gunthorpe
2019-06-20 16:12 ` [RFC PATCH 17/28] block: Introduce queue flag to indicate support for dma-direct bios Logan Gunthorpe
2019-06-20 16:12 ` [RFC PATCH 18/28] block: Introduce bio_add_dma_addr() Logan Gunthorpe
2019-06-20 16:12 ` [RFC PATCH 19/28] nvme-pci: Support dma-direct bios Logan Gunthorpe
2019-06-20 16:12 ` [RFC PATCH 20/28] IB/core: Introduce API for initializing a RW ctx from a DMA address Logan Gunthorpe
2019-06-20 16:49 ` Jason Gunthorpe
2019-06-20 16:59 ` Logan Gunthorpe
2019-06-20 17:11 ` Jason Gunthorpe
2019-06-20 18:24 ` Logan Gunthorpe
2019-06-20 16:12 ` [RFC PATCH 21/28] nvmet: Split nvmet_bdev_execute_rw() into a helper function Logan Gunthorpe
2019-06-20 16:12 ` [RFC PATCH 22/28] nvmet: Use DMA addresses instead of struct pages for P2P Logan Gunthorpe
2019-06-20 16:12 ` [RFC PATCH 23/28] nvme-pci: Remove support for PCI_P2PDMA requests Logan Gunthorpe
2019-06-20 16:12 ` [RFC PATCH 24/28] block: Remove PCI_P2PDMA queue flag Logan Gunthorpe
2019-06-20 16:12 ` [RFC PATCH 25/28] IB/core: Remove P2PDMA mapping support in rdma_rw_ctx Logan Gunthorpe
2019-06-20 16:12 ` [RFC PATCH 26/28] PCI/P2PDMA: Remove SGL helpers Logan Gunthorpe
2019-06-20 16:12 ` [RFC PATCH 27/28] PCI/P2PDMA: Remove struct pages that back P2PDMA memory Logan Gunthorpe
2019-06-20 16:12 ` [RFC PATCH 28/28] memremap: Remove PCI P2PDMA page memory type Logan Gunthorpe
2019-06-20 18:45 ` [RFC PATCH 00/28] Removing struct page from P2PDMA Dan Williams
2019-06-20 19:33 ` Jason Gunthorpe
2019-06-20 20:18 ` Dan Williams
2019-06-20 20:51 ` Logan Gunthorpe
2019-06-21 17:47 ` Jason Gunthorpe
2019-06-21 17:54 ` Dan Williams
2019-06-24 7:31 ` Christoph Hellwig
2019-06-24 13:46 ` Jason Gunthorpe
2019-06-24 13:50 ` Christoph Hellwig
2019-06-24 13:55 ` Jason Gunthorpe
2019-06-24 16:53 ` Logan Gunthorpe
2019-06-24 18:16 ` Jason Gunthorpe
2019-06-24 18:28 ` Logan Gunthorpe
2019-06-24 18:54 ` Jason Gunthorpe
2019-06-24 19:37 ` Logan Gunthorpe
2019-06-24 16:10 ` Logan Gunthorpe
2019-06-25 7:18 ` Christoph Hellwig
2019-06-20 19:34 ` Logan Gunthorpe
2019-06-20 23:40 ` Dan Williams
2019-06-20 23:42 ` Logan Gunthorpe
2019-06-24 7:27 ` Christoph Hellwig
2019-06-24 16:07 ` Logan Gunthorpe [this message]
2019-06-25 7:20 ` Christoph Hellwig
2019-06-25 15:57 ` Logan Gunthorpe
2019-06-25 17:01 ` Christoph Hellwig
2019-06-25 19:54 ` Logan Gunthorpe
2019-06-26 6:57 ` Christoph Hellwig
2019-06-26 18:31 ` Logan Gunthorpe
2019-06-26 20:21 ` Jason Gunthorpe
2019-06-26 20:39 ` Dan Williams
2019-06-26 20:54 ` Jason Gunthorpe
2019-06-26 20:55 ` Logan Gunthorpe
2019-06-26 20:45 ` Logan Gunthorpe
2019-06-26 21:00 ` Jason Gunthorpe
2019-06-26 21:18 ` Logan Gunthorpe
2019-06-27 6:32 ` Jason Gunthorpe
2019-06-27 16:09 ` Logan Gunthorpe
2019-06-27 16:35 ` Jason Gunthorpe
2019-06-27 16:49 ` Logan Gunthorpe
2019-06-28 4:57 ` Jason Gunthorpe
2019-06-28 16:22 ` Logan Gunthorpe
2019-06-28 17:29 ` Jason Gunthorpe
2019-06-28 18:29 ` Logan Gunthorpe
2019-06-28 19:09 ` Jason Gunthorpe
2019-06-28 19:35 ` Logan Gunthorpe
2019-07-02 22:45 ` Jason Gunthorpe
2019-07-02 22:52 ` Logan Gunthorpe
2019-06-27 9:08 ` Christoph Hellwig
2019-06-27 16:30 ` Logan Gunthorpe
2019-06-27 17:00 ` Christoph Hellwig
2019-06-27 18:00 ` Logan Gunthorpe
2019-06-28 13:38 ` Christoph Hellwig
2019-06-28 15:54 ` Logan Gunthorpe
2019-06-27 9:01 ` Christoph Hellwig
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