linux-kernel - Re: [PATCH 09/12] nvme-pci: Use PCI p2pmem subsystem to manage the CMB

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Date:   Fri, 5 Jan 2018 12:04:05 -0700
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Keith Busch <keith.busch@...el.com>
Cc:     linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-rdma@...r.kernel.org,
        linux-nvdimm@...ts.01.org, linux-block@...r.kernel.org,
        Stephen Bates <sbates@...thlin.com>,
        Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
        Sagi Grimberg <sagi@...mberg.me>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Max Gurtovoy <maxg@...lanox.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [PATCH 09/12] nvme-pci: Use PCI p2pmem subsystem to manage the
 CMB



On 05/01/18 12:01 PM, Keith Busch wrote:
> On Fri, Jan 05, 2018 at 11:19:28AM -0700, Logan Gunthorpe wrote:
>> Although it is not explicitly stated anywhere, pci_alloc_p2pmem() should
>> always be at least 4k aligned. This is because the gen_pool that implements
>> it is created with PAGE_SHIFT for its min_alloc_order.
> 
> Ah, I see that now. Thanks for the explanation.
> 
> Does it need to be created with page sized minimum alloc order? That
> granularity makes it difficult to fit SQs in CMB on archs with larger
> pages when we only needed 4k alignment.
> 
> I was also hoping to extend this for PRP/SGL in CMB where even 4k is
> too high a granularity to make it really useful.
> 
> It looks like creating the gen pool with a smaller minimum and
> gen_pool_first_fit_order_align algo would satisfy my use cases, but I'm
> not sure if there's another reason you've set it to page alignment.

I don't see any reason why we couldn't change this at some point if it 
makes sense to. PAGE_SIZE just seemed like a suitable choice but I don't 
think anything depends on it. In the end, I guess it depends on how 
limited typical CMB buffers end up being.

Logan