Re: [PATCH rdma-next 3/4] lib/scatterlist: Add support in dynamic allocation of SG table from pages

From: Christoph Hellwig <hch@lst.de>
To: Maor Gottlieb <maorg@nvidia.com>
Cc: Christoph Hellwig <hch@lst.de>, Leon Romanovsky <leon@kernel.org>,
	Doug Ledford <dledford@redhat.com>,
	Jason Gunthorpe <jgg@nvidia.com>,
	linux-kernel@vger.kernel.org, linux-rdma@vger.kernel.org
Subject: Re: [PATCH rdma-next 3/4] lib/scatterlist: Add support in dynamic allocation of SG table from pages
Date: Tue, 8 Sep 2020 17:54:09 +0200	[thread overview]
Message-ID: <20200908155409.GB13593@lst.de> (raw)
In-Reply-To: <15552707-c9ae-b76b-f6ff-7fedd5b02aed@nvidia.com>

On Mon, Sep 07, 2020 at 03:44:08PM +0300, Maor Gottlieb wrote:
>>> +{
>>> +#ifdef CONFIG_ARCH_NO_SG_CHAIN
>>> +	if (append->left_pages)
>>> +		return ERR_PTR(-EOPNOTSUPP);
>>> +#endif
>> Which makes this API entirely useless for !CONFIG_ARCH_NO_SG_CHAIN,
>> doesn't it?  Wouldn't it make more sense to not provide it for that
>> case and add an explicitl dependency in the callers?
>
> Current implementation allow us to support small memory registration which 
> not require chaining. I am not aware which archs has the SG_CHAIN support 
> and I don't want to break it so I can't add it to as dependency to the 
> Kconfig. Another option is to do the logic in the caller, but it isn't 
> clean.

But does the caller handle the -EOPNOTSUPP properly?  I think right now
it will just fail the large registration that worked before this patchset.

Given that ARCH_NO_SG_CHAIN is only true for alpha, parisc and a few
arm subarchitectures I think just not supporting umem is probably
cleared.  And eventually we'll need to drop ARCH_NO_SG_CHAIN entirely.