linux-kernel - Re: Phyr Starter

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Message-ID: <305b0b3b-e5d1-3dc2-a4a3-01c05dda6748@deltatee.com>
Date:   Tue, 11 Jan 2022 10:31:42 -0700
From:   Logan Gunthorpe <logang@...tatee.com>
To:     John Hubbard <jhubbard@...dia.com>,
        Matthew Wilcox <willy@...radead.org>,
        linux-kernel@...r.kernel.org
Cc:     Christoph Hellwig <hch@....de>, Jason Gunthorpe <jgg@...dia.com>,
        Joao Martins <joao.m.martins@...cle.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 2022-01-11 1:17 a.m., John Hubbard wrote:
> On 1/10/22 11:34, Matthew Wilcox wrote:
>> TLDR: I want to introduce a new data type:
>>
>> struct phyr {
>>          phys_addr_t addr;
>>          size_t len;
>> };
>>
>> and use it to replace bio_vec as well as using it to replace the array
>> of struct pages used by get_user_pages() and friends.
>>
>> ---
> 
> This would certainly solve quite a few problems at once. Very compelling.

I agree.

> 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*().
> 
> And changing the bio_vec for *that* purpose was not really acceptable.
> 
> But now that you're looking to change it in a big way (and with some
> spare bits avaiable...oohh!), maybe I can go that direction after all.
> 
> Or, are you looking at a design in which any phyr is implicitly FOLL_PIN'd
> if it exists at all?

I'd also second being able to store a handful of flags in each phyr. My
userspace P2PDMA patchset needs to add a flag to each sgl to indicate
whether it was mapped as a bus address or not (which would be necessary
for the DMA mapped side dma_map_phyr).

Though, it's not immediately obvious where to put the flags without
increasing the size of the structure :(

Logan