linux-kernel - Re: [PATCH v2 2/4] mm/gup: handle ZONE_DEVICE pages in folio_fast_pin

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Date: Wed, 26 Jun 2024 16:23:12 -0600
From: Martin Oliveira <martin.oliveira@...eticom.com>
To: John Hubbard <jhubbard@...dia.com>, linux-rdma@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc: Jason Gunthorpe <jgg@...pe.ca>, Leon Romanovsky <leon@...nel.org>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Tejun Heo <tj@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Logan Gunthorpe <logang@...tatee.com>,
 Mike Marciniszyn <mike.marciniszyn@...el.com>,
 Shiraz Saleem <shiraz.saleem@...el.com>,
 Michael Guralnik <michaelgur@...dia.com>,
 Artemy Kovalyov <artemyko@...dia.com>, david.sloan@...eticom.com
Subject: Re: [PATCH v2 2/4] mm/gup: handle ZONE_DEVICE pages in
 folio_fast_pin_allowed()

Hi John,

Thanks for your comments and sorry for the delayed response, I was off the
last few days.

On 2024-06-14 20:40, John Hubbard wrote:
> s/folio_fast_pin_allowed/gup_fast_folio_allowed/ ?

Nice catch! That function got renamed from the original work
we did on this series.

> This comment is very difficult, because it states that one cannot
> do something, right before explicitly enable something else. And the
> reader is given little help on connecting the two.
> 
> And there are several subtypes of ZONE_DEVICE. Is it really true that
> none of them can be mapped to user space? For p2p BAR1 mappings, those
> actually go to user space, yes? Confused, need help. :)
This is a fair point, I had only looked at p2p but I can't say anything
about the other subtypes of ZONE_DEVICE.

For p2p, yes, they will go to userspace, however folio->mapping was NULL
for those cases. And hence fast path would reject it.

In any case, I think we could drop this patch for now and revisit when/if
required, the regular gup path was not measurably slower.

Thanks,
Martin