linux-kernel - Re: [PATCH v1 0/4] smaps / mm/gup: fix gup_can_follow

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Date:   Fri, 28 Jul 2023 15:00:04 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Peter Xu <peterx@...hat.com>
CC:     David Hildenbrand <david@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        <linux-fsdevel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        liubo <liubo254@...wei.com>,
        Matthew Wilcox <willy@...radead.org>,
        Hugh Dickins <hughd@...gle.com>, Jason Gunthorpe <jgg@...pe.ca>
Subject: Re: [PATCH v1 0/4] smaps / mm/gup: fix gup_can_follow_protnone
 fallout

On 7/28/23 14:49, Peter Xu wrote:
>> The story of how FOLL_NUMA and FOLL_FORCE became entangled was enlightening,
>> by the way, and now that I've read it I don't want to go back. :)
> 
> Yeah I fully agree we should hopefully remove the NUMA / FORCE
> tangling.. even if we want to revert back to the FOLL_NUMA flag we may want
> to not revive that specific part.  I had a feeling that we're all on the
> same page there.
> 

Yes, I think so. :)

> It's more about the further step to make FOLL_NUMA opt-in for GUP.

Let's say "FOLL_HONOR_NUMA_FAULT" for this next discussion, but yes. So
given that our API allows passing in FOLL_ flags, I don't understand the
objection to letting different callers pass in, or not pass in, that
flag.

It's the perfect way to clean up the whole thing. As Linus suggested
slightly earlier here, there can be a comment at the call site,
explaining why KVM needs FOLL_HONOR_NUMA_FAULT, and you're good, right?


thanks,
-- 
John Hubbard
NVIDIA