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

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Message-ID: <ZMQ32RRJlW/aDYAE@x1n>
Date:   Fri, 28 Jul 2023 17:49:13 -0400
From:   Peter Xu <peterx@...hat.com>
To:     John Hubbard <jhubbard@...dia.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

Hello, John,

On Fri, Jul 28, 2023 at 02:32:12PM -0700, John Hubbard wrote:
> 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. :)

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.

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

Thanks,

-- 
Peter Xu