linux-kernel - Re: [PATCH rdma-next 4/4] RDMA/umem: Move to allocate SG table from pages

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Date:   Mon, 7 Sep 2020 09:29:26 +0200
From:   Christoph Hellwig <hch@....de>
To:     Leon Romanovsky <leon@...nel.org>
Cc:     Christoph Hellwig <hch@....de>, Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Maor Gottlieb <maorg@...dia.com>, linux-kernel@...r.kernel.org,
        linux-rdma@...r.kernel.org
Subject: Re: [PATCH rdma-next 4/4] RDMA/umem: Move to allocate SG table
 from pages

On Thu, Sep 03, 2020 at 03:18:53PM +0300, Leon Romanovsky wrote:
> From: Maor Gottlieb <maorg@...dia.com>
> 
> Remove the implementation of ib_umem_add_sg_table and instead
> call to sg_alloc_table_append which already has the logic to
> merge contiguous pages.
> 
> Besides that it removes duplicated functionality, it reduces the
> memory consumption of the SG table significantly. Prior to this
> patch, the SG table was allocated in advance regardless consideration
> of contiguous pages.
> 
> In huge pages system of 2MB page size, without this change, the SG table
> would contain x512 SG entries.
> E.g. for 100GB memory registration:
> 
> 	 Number of entries	Size
> Before 	      26214400          600.0MB
> After            51200		  1.2MB
> 
> Signed-off-by: Maor Gottlieb <maorg@...dia.com>
> Signed-off-by: Leon Romanovsky <leonro@...dia.com>

Looks sensible for now, but the real fix is of course to avoid
the scatterlist here entirely, and provide a bvec based
pin_user_pages_fast.  I'll need to finally get that done..