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Date:   Thu, 10 Sep 2020 20:21:06 -0300
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Gerald Schaefer <gerald.schaefer@linux.ibm.com>
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Subject: Re: [RFC PATCH v2 1/3] mm/gup: fix gup_fast with dynamic page table
 folding
Message-ID: <20200910232106.GR87483@ziepe.ca>
References: <20200907180058.64880-1-gerald.schaefer@linux.ibm.com>
 <20200907180058.64880-2-gerald.schaefer@linux.ibm.com>
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 <20200909142904.00b72921@thinkpad>
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 <20200909192534.442f8984@thinkpad>
 <20200909180324.GI87483@ziepe.ca>
 <20200910093925.GB29166@oc3871087118.ibm.com>
 <20200910130233.GK87483@ziepe.ca>
 <20200910195749.795232d1@thinkpad>
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On Thu, Sep 10, 2020 at 07:57:49PM +0200, Gerald Schaefer wrote:
> On Thu, 10 Sep 2020 10:02:33 -0300
> Jason Gunthorpe <jgg@ziepe.ca> wrote:
> 
> > On Thu, Sep 10, 2020 at 11:39:25AM +0200, Alexander Gordeev wrote:
> > 
> > > As Gerald mentioned, it is very difficult to explain in a clear way.
> > > Hopefully, one could make sense ot of it.
> > 
> > I would say the page table API requires this invariant:
> > 
> >         pud = pud_offset(p4d, addr);
> >         do {
> > 		WARN_ON(pud != pud_offset(p4d, addr);
> >                 next = pud_addr_end(addr, end);
> >         } while (pud++, addr = next, addr != end);
> > 
> > ie pud++ is supposed to be a shortcut for 
> >   pud_offset(p4d, next)
> > 
> 
> Hmm, IIUC, all architectures with static folding will simply return
> the passed-in p4d pointer for pud_offset(p4d, addr), for 3-level
> pagetables.

It is probably moot now, but since other arch's don't crash they also
return pud_addr_end() == end so the loop only does one iteration.

ie pud == pud_offset(p4d, addr) for all iterations as the pud++ never
happens.

Which is what this addr_end patch does for s390..

Jason