netdev - Re: Phyr Starter

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Message-ID: <a6b65260-669b-65d8-c20f-0d75e0393200@nvidia.com>
Date:   Thu, 20 Jan 2022 13:35:23 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Matthew Wilcox <willy@...radead.org>, linux-kernel@...r.kernel.org,
        Jason Gunthorpe <jgg@...dia.com>,
        Joao Martins <joao.m.martins@...cle.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Ming Lei <ming.lei@...hat.com>, linux-block@...r.kernel.org,
        netdev@...r.kernel.org, linux-mm@...ck.org,
        linux-rdma@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        nvdimm@...ts.linux.dev
Subject: Re: Phyr Starter

On 1/20/22 6:12 AM, Christoph Hellwig wrote:
> On Tue, Jan 11, 2022 at 12:17:18AM -0800, John Hubbard wrote:
>> Zooming in on the pinning aspect for a moment: last time I attempted to
>> convert O_DIRECT callers from gup to pup, I recall wanting very much to
>> record, in each bio_vec, whether these pages were acquired via FOLL_PIN,
>> or some non-FOLL_PIN method. Because at the end of the IO, it is not
>> easy to disentangle which pages require put_page() and which require
>> unpin_user_page*().
> 
> I don't think that is a problem.  Pinning only need to happen for
> ITER_IOVEC, and the only non-user pages there is the ZERO_PAGE added
> for padding that can be special cased.

I am really glad to hear you say that. Because I just worked through it
again in detail yesterday (including your and others' old emails about
this), and tentatively reached the same conclusion from seeing the call 
paths. But I wanted to confirm with someone who actually knows this code 
well, and that's not me. :)

Things like dio_refill_pages() are mixing in the zero page, but like you 
say, that can be handled. I have a few ideas for that.

Now that the goal is a considerably narrower as compared to in 2019 
("convert DIO callers to pup", instead of "convert the world to pup", 
ha), this looks quite feasible after all.

thanks,
-- 
John Hubbard
NVIDIA