linux-kernel - Re: [PATCH 05/12] net: mana: Set the DMA device max page size

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Date:   Tue, 17 May 2022 21:03:56 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Long Li <longli@...rosoft.com>
Cc:     Ajay Sharma <sharmaajay@...rosoft.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Leon Romanovsky <leon@...nel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>
Subject: Re: [PATCH 05/12] net: mana: Set the DMA device max page size

On Tue, May 17, 2022 at 08:04:58PM +0000, Long Li wrote:
> > Subject: Re: [PATCH 05/12] net: mana: Set the DMA device max page size
> > 
> > On Tue, May 17, 2022 at 07:32:51PM +0000, Long Li wrote:
> > > > Subject: Re: [PATCH 05/12] net: mana: Set the DMA device max page
> > > > size
> > > >
> > > > On Tue, May 17, 2022 at 02:04:29AM -0700, longli@...uxonhyperv.com
> > wrote:
> > > > > From: Long Li <longli@...rosoft.com>
> > > > >
> > > > > The system chooses default 64K page size if the device does not
> > > > > specify the max page size the device can handle for DMA. This do
> > > > > not work well when device is registering large chunk of memory in
> > > > > that a large page size is more efficient.
> > > > >
> > > > > Set it to the maximum hardware supported page size.
> > > >
> > > > For RDMA devices this should be set to the largest segment size an
> > > > ib_sge can take in when posting work. It should not be the page size
> > > > of MR. 2M is a weird number for that, are you sure it is right?
> > >
> > > Yes, this is the maximum page size used in hardware page tables.
> > 
> > As I said, it should be the size of the sge in the WQE, not the "hardware page
> > tables"
> 
> This driver uses the following code to figure out the largest page
> size for memory registration with hardware:
> 
> page_sz = ib_umem_find_best_pgsz(mr->umem, PAGE_SZ_BM, iova);
> 
> In this function, mr->umem is created with ib_dma_max_seg_size() as
> its max segment size when creating its sgtable.
>
> The purpose of setting DMA page size to 2M is to make sure this
> function returns the largest possible MR size that the hardware can
> take. Otherwise, this function will return 64k: the default DMA
> size.

As I've already said, you are supposed to set the value that limits to
ib_sge and *NOT* the value that is related to
ib_umem_find_best_pgsz. It is usually 2G because the ib_sge's
typically work on a 32 bit length.

Jason