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

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Message-ID: <20200908155409.GB13593@lst.de>
Date:   Tue, 8 Sep 2020 17:54:09 +0200
From:   Christoph Hellwig <hch@....de>
To:     Maor Gottlieb <maorg@...dia.com>
Cc:     Christoph Hellwig <hch@....de>, Leon Romanovsky <leon@...nel.org>,
        Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...dia.com>, linux-kernel@...r.kernel.org,
        linux-rdma@...r.kernel.org
Subject: Re: [PATCH rdma-next 3/4] lib/scatterlist: Add support in dynamic
 allocation of SG table from pages

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.