linux-kernel - Re: [PATCH v4 0/2] block: Enable proper MMIO memory handling for P2P DMA

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Message-ID: <4f75497d-11cb-437c-ab90-d65d4d2e0a52@kernel.dk>
Date: Thu, 13 Nov 2025 10:45:53 -0700
From: Jens Axboe <axboe@...nel.dk>
To: Keith Busch <kbusch@...nel.org>, Christoph Hellwig <hch@....de>,
 Sagi Grimberg <sagi@...mberg.me>, Leon Romanovsky <leon@...nel.org>
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-nvme@...ts.infradead.org
Subject: Re: [PATCH v4 0/2] block: Enable proper MMIO memory handling for P2P
 DMA

On 11/13/25 10:12 AM, Jens Axboe wrote:
> On 11/13/25 9:39 AM, Jens Axboe wrote:
>>
>> On Wed, 12 Nov 2025 21:48:03 +0200, Leon Romanovsky wrote:
>>> Changelog:
>>> v4:
>>>  * Changed double "if" to be "else if".
>>>  * Added missed PCI_P2PDMA_MAP_NONE case.
>>> v3: https://patch.msgid.link/20251027-block-with-mmio-v3-0-ac3370e1f7b7@nvidia.com
>>>  * Encoded p2p map type in IOD flags instead of DMA attributes.
>>>  * Removed REQ_P2PDMA flag from block layer.
>>>  * Simplified map_phys conversion patch.
>>> v2: https://lore.kernel.org/all/20251020-block-with-mmio-v2-0-147e9f93d8d4@nvidia.com/
>>>  * Added Chirstoph's Reviewed-by tag for first patch.
>>>  * Squashed patches
>>>  * Stored DMA MMIO attribute in NVMe IOD flags variable instead of block layer.
>>> v1: https://patch.msgid.link/20251017-block-with-mmio-v1-0-3f486904db5e@nvidia.com
>>>  * Reordered patches.
>>>  * Dropped patch which tried to unify unmap flow.
>>>  * Set MMIO flag separately for data and integrity payloads.
>>> v0: https://lore.kernel.org/all/cover.1760369219.git.leon@kernel.org/
>>>
>>> [...]
>>
>> Applied, thanks!
>>
>> [1/2] nvme-pci: migrate to dma_map_phys instead of map_page
>>       commit: f10000db2f7cf29d8c2ade69266bed7b51c772cb
>> [2/2] block-dma: properly take MMIO path
>>       commit: 8df2745e8b23fdbe34c5b0a24607f5aaf10ed7eb
> 
> And now dropped again - this doesn't boot on neither my big test box
> with 33 nvme drives, nor even on my local test vm. Two different archs,
> and very different setups. Which begs the question, how on earth was
> this tested, if it doesn't boot on anything I have here?!

I took a look, and what happens here is that iter.p2pdma.map is 0 as it
never got set to anything. That is the same as PCI_P2PDMA_MAP_UNKNOWN,
and hence we just end up in a BLK_STS_RESOURCE. First of all, returning
BLK_STS_RESOURCE for that seems... highly suspicious. That should surely
be a fatal error. And secondly, this just further backs up that there's
ZERO testing done on this patchset at all. WTF?

FWIW, the below makes it boot just fine, as expected, as a default zero
filled iter then matches the UNKNOWN case.


diff --git a/drivers/nvme/host/pci.c b/drivers/nvme/host/pci.c
index e5ca8301bb8b..4cce69226773 100644
--- a/drivers/nvme/host/pci.c
+++ b/drivers/nvme/host/pci.c
@@ -1087,6 +1087,7 @@ static blk_status_t nvme_map_data(struct request *req)
 	case PCI_P2PDMA_MAP_THRU_HOST_BRIDGE:
 		iod->flags |= IOD_DATA_MMIO;
 		break;
+	case PCI_P2PDMA_MAP_UNKNOWN:
 	case PCI_P2PDMA_MAP_NONE:
 		break;
 	default:
@@ -1122,6 +1123,7 @@ static blk_status_t nvme_pci_setup_meta_iter(struct request *req)
 	case PCI_P2PDMA_MAP_THRU_HOST_BRIDGE:
 		iod->flags |= IOD_META_MMIO;
 		break;
+	case PCI_P2PDMA_MAP_UNKNOWN:
 	case PCI_P2PDMA_MAP_NONE:
 		break;
 	default:

-- 
Jens Axboe