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

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Message-ID: <edd9b468-2d60-1df7-a515-22475fd94fe2@nvidia.com>
Date:   Fri, 28 Jul 2023 14:32:12 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Peter Xu <peterx@...hat.com>, David Hildenbrand <david@...hat.com>
CC:     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 14:20, Peter Xu wrote:
> On Fri, Jul 28, 2023 at 11:02:46PM +0200, David Hildenbrand wrote:
>> Can we get a simple revert in first (without that FOLL_FORCE special casing
>> and ideally with a better name) to handle stable backports, and I'll
>> follow-up with more documentation and letting GUP callers pass in that flag
>> instead?
>>
>> That would help a lot. Then we also have more time to let that "move it to
>> GUP callers" mature a bit in -next, to see if we find any surprises?
> 
> As I raised my concern over the other thread, I still worry numa users can
> be affected by this change. After all, numa isn't so uncommon to me, at
> least fedora / rhel as CONFIG_NUMA_BALANCING_DEFAULT_ENABLED=y. I highly
> suspect that's also true to major distros.  Meanwhile all kernel modules
> use gup..
> 
> I'd say we can go ahead and try if we want, but I really don't know why
> that helps in any form to move it to the callers.. with the risk of
> breaking someone.

It's worth the trouble, in order to clear up this historical mess. It's
helping *future* callers of the API, and future maintenance efforts. Yes
there is some risk, but it seems very manageable.

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. :)


thanks,
-- 
John Hubbard
NVIDIA