From: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
To: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Peter Zijlstra <peterz@infradead.org>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Dave Hansen <dave.hansen@linux.intel.com>,
Dave Hansen <dave.hansen@intel.com>,
Paul Mackerras <paulus@samba.org>,
linux-sparc <sparclinux@vger.kernel.org>,
Alexander Gordeev <agordeev@linux.ibm.com>,
Claudio Imbrenda <imbrenda@linux.ibm.com>,
Will Deacon <will@kernel.org>,
linux-arch <linux-arch@vger.kernel.org>,
linux-s390 <linux-s390@vger.kernel.org>,
Vasily Gorbik <gor@linux.ibm.com>,
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Russell King <linux@armlinux.org.uk>,
Christian Borntraeger <borntraeger@de.ibm.com>,
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Mike Rapoport <rppt@kernel.org>
Subject: Re: [RFC PATCH v2 1/3] mm/gup: fix gup_fast with dynamic page table folding
Date: Thu, 10 Sep 2020 19:57:49 +0200 [thread overview]
Message-ID: <20200910195749.795232d1@thinkpad> (raw)
In-Reply-To: <20200910130233.GK87483@ziepe.ca>
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. There is no difference for s390. For gup_fast, that p4d
pointer is not really a pointer to a value in a pagetable, but
to some local copy of such a value, and not just for s390.
So, pud = p4d = pointer to copy, and increasing that pud pointer
cannot be the same as pud_offset(p4d, next). I do see your point
however, at last I think :-) My problem is that I do not see where
we would have an s390-specific issue here. Maybe my understanding
of how it works for others with static folding is wrong. That
would explain my difficulties in getting your point...
> While S390 does not follow this. Fixing addr_end brings it into
> alignment by preventing pud++ from happening.
Exactly, only that nobody seems to follow it, IIUC. Fixing it up
with pXd_addr_end was my impression of what we need to do, in order to
have it work the same way as for others.
> The only currently known side effect is that gup_fast crashes, but it
> sure is an unexpected thing.
Well, from my understanding it feels more unexpected that something
that is supposed to be a pointer to an entry in a page table, really is
just a pointer to some copy somewhere.
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next prev parent reply other threads:[~2020-09-10 18:03 UTC|newest]
Thread overview: 62+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-07 18:00 [RFC PATCH v2 0/3] mm/gup: fix gup_fast with dynamic page table folding Gerald Schaefer
2020-09-07 18:00 ` [RFC PATCH v2 1/3] " Gerald Schaefer
2020-09-08 5:06 ` Christophe Leroy
2020-09-08 12:09 ` Christian Borntraeger
2020-09-08 12:40 ` Christophe Leroy
2020-09-08 13:38 ` Gerald Schaefer
2020-09-08 14:30 ` Dave Hansen
2020-09-08 17:59 ` Gerald Schaefer
2020-09-09 12:29 ` Gerald Schaefer
2020-09-09 16:18 ` Dave Hansen
2020-09-09 17:25 ` Gerald Schaefer
2020-09-09 18:03 ` Jason Gunthorpe
2020-09-10 9:39 ` Alexander Gordeev
2020-09-10 13:02 ` Jason Gunthorpe
2020-09-10 13:28 ` Gerald Schaefer
2020-09-10 15:10 ` Jason Gunthorpe
2020-09-10 17:07 ` Gerald Schaefer
2020-09-10 17:19 ` Jason Gunthorpe
2020-09-10 17:57 ` Gerald Schaefer [this message]
2020-09-10 23:21 ` Jason Gunthorpe
2020-09-10 17:35 ` Linus Torvalds
2020-09-10 18:13 ` Jason Gunthorpe
2020-09-10 18:33 ` Linus Torvalds
2020-09-10 19:10 ` Gerald Schaefer
2020-09-10 19:32 ` Linus Torvalds
2020-09-10 21:59 ` Jason Gunthorpe
2020-09-11 7:09 ` peterz
2020-09-11 11:19 ` Jason Gunthorpe
2020-09-11 19:03 ` [PATCH] " Vasily Gorbik
2020-09-11 19:09 ` Linus Torvalds
2020-09-11 19:40 ` Jason Gunthorpe
2020-09-11 20:05 ` Jason Gunthorpe
2020-09-11 20:36 ` [PATCH v2] " Vasily Gorbik
2020-09-15 17:09 ` Vasily Gorbik
2020-09-15 17:14 ` Jason Gunthorpe
2020-09-15 17:18 ` Mike Rapoport
2020-09-15 17:31 ` John Hubbard
2020-09-10 21:22 ` [RFC PATCH v2 1/3] " John Hubbard
2020-09-10 22:11 ` Jason Gunthorpe
2020-09-10 22:17 ` John Hubbard
2020-09-11 12:19 ` Alexander Gordeev
2020-09-11 16:45 ` Linus Torvalds
2020-09-10 13:11 ` Gerald Schaefer
2020-09-07 18:00 ` [RFC PATCH v2 2/3] mm: make pXd_addr_end() functions page-table entry aware Gerald Schaefer
2020-09-08 5:14 ` Christophe Leroy
2020-09-08 7:46 ` Alexander Gordeev
2020-09-08 8:16 ` Christophe Leroy
2020-09-08 14:15 ` Alexander Gordeev
2020-09-09 8:38 ` Christophe Leroy
2020-09-08 14:25 ` Alexander Gordeev
2020-09-08 13:26 ` Jason Gunthorpe
2020-09-08 14:33 ` Dave Hansen
2020-09-07 18:00 ` [RFC PATCH v2 3/3] mm: make generic pXd_addr_end() macros inline functions Gerald Schaefer
2020-09-07 20:15 ` Mike Rapoport
2020-09-08 5:19 ` Christophe Leroy
2020-09-08 15:48 ` Alexander Gordeev
2020-09-08 17:20 ` Christophe Leroy
2020-09-07 20:12 ` [RFC PATCH v2 0/3] mm/gup: fix gup_fast with dynamic page table folding Mike Rapoport
2020-09-08 5:22 ` Christophe Leroy
2020-09-08 17:36 ` Gerald Schaefer
2020-09-09 16:12 ` Gerald Schaefer
2020-09-08 4:42 ` Christophe Leroy
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