Re: [PATCH rdma-next 1/4] lib/scatterlist: Refactor sg_alloc_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 1/4] lib/scatterlist: Refactor sg_alloc_table_from_pages
Date: Tue, 8 Sep 2020 17:52:28 +0200	[thread overview]
Message-ID: <20200908155228.GA13593@lst.de> (raw)
In-Reply-To: <2a4b0211-c7a0-2a82-1335-7ed935b92aa2@nvidia.com>

On Mon, Sep 07, 2020 at 03:32:31PM +0300, Maor Gottlieb wrote:
>
> On 9/7/2020 10:29 AM, Christoph Hellwig wrote:
>> On Thu, Sep 03, 2020 at 06:54:34PM +0300, Leon Romanovsky wrote:
>>> From: Maor Gottlieb <maorg@nvidia.com>
>>>
>>> Currently, sg_alloc_table_from_pages doesn't support dynamic chaining of
>>> SG entries. Therefore it requires from user to allocate all the pages in
>>> advance and hold them in a large buffer. Such a buffer consumes a lot of
>>> temporary memory in HPC systems which do a very large memory registration.
>>>
>>> The next patches introduce API for dynamically allocation from pages and
>>> it requires us to do the following:
>>>   * Extract the code to alloc_from_pages_common.
>>>   * Change the build of the table to iterate on the chunks and not on the
>>>     SGEs. It will allow dynamic allocation of more SGEs.
>>>
>>> Since sg_alloc_table_from_pages allocate exactly the number of chunks,
>>> therefore chunks are equal to the number of SG entries.
>> Given how few users __sg_alloc_table_from_pages has, what about just
>> switching it to your desired calling conventions without another helper?
>
> I tried it now. It didn't save a lot.  Please give me your decision and if 
> needed I will update accordingly.

Feel free to keep it for now, we can sort this out later.