linux-kernel - Re: [PATCH rdma-next 1/4] lib/scatterlist: Refactor sg_alloc_table_from

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Date:   Tue, 8 Sep 2020 17:52:28 +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 1/4] lib/scatterlist: Refactor
 sg_alloc_table_from_pages

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@...dia.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.