From: Martin Schwidefsky <schwidefsky@de.ibm.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Christoph Hellwig <hch@infradead.org>,
Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
Michael Ellerman <mpe@ellerman.id.au>,
linuxppc-dev@lists.ozlabs.org,
linux-s390 <linux-s390@vger.kernel.org>
Subject: Re: Linux 5.1-rc5
Date: Thu, 18 Apr 2019 20:41:44 +0200 [thread overview]
Message-ID: <20190418204144.16adf2a0@mschwideX1> (raw)
In-Reply-To: <CAHk-=whajv8Fw2VcBqZUs0hfx1Dq4fYj3TEEx4yfR3E8+rLS1A@mail.gmail.com>
On Thu, 18 Apr 2019 08:49:32 -0700
Linus Torvalds <torvalds@linux-foundation.org> wrote:
> On Thu, Apr 18, 2019 at 1:02 AM Martin Schwidefsky
> <schwidefsky@de.ibm.com> wrote:
> >
> > The problematic lines in the generic gup code are these three:
> >
> > 1845: pmdp = pmd_offset(&pud, addr);
> > 1888: pudp = pud_offset(&p4d, addr);
> > 1916: p4dp = p4d_offset(&pgd, addr);
> >
> > Passing the pointer of a *copy* of a page table entry to pxd_offset() does
> > not work with the page table folding on s390.
>
> Hmm. I wonder why. x86 too does the folding thing for the p4d and pud case.
>
> The folding works with the local copy just the same way it works with
> the orignal value.
The difference is that with the static page table folding pgd_offset()
does the index calculation of the actual hardware top-level table. With
dynamic page table folding as s390 is doing it, if the task does not use
a 5-level page table pgd_offset() will see a pgd_index() of 0, the indexing
of the actual top-level table is done later with p4d_offset(), pud_offset()
or pmd_offset().
As an example, with a three level page table we have three indexes x/y/z.
The common code "thinks" 5 indexing steps, with static folding the index
sequence is x 0 0 y z. With dynamic folding the sequence is 0 0 x y z.
By moving the first indexing operation to pgd_offset the static sequence
does not add an index to a non-dereferenced pointer to a stack variable,
the dynamic sequence does.
> But I see that s390 does some other kind of folding and does that
> addition of the p*d_index() unconditionally.
>
> I guess that does mean that s390 will just have to have its own walker.
>
> For the issue of the page refcount overflow it really isn't a huge
> deal. Adding the refcount checking is simple (see the example patch I
> gave for powerpc - you'll just have a couple of extra cases since you
> do it all, rather than just the special hugetlb cases).
>
> Obviously in general it would have been nicer to share as much code as
> possible, but let's not make things unnecessarily complex if s390 is
> just fundamentally different..
It would have been nice to use the generic code (less bugs) but not at
the price of over-complicating things. And that page table folding thing
always makes my head hurt.
--
blue skies,
Martin.
"Reality continues to ruin my life." - Calvin.
next prev parent reply other threads:[~2019-04-18 18:41 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-14 22:40 Linux 5.1-rc5 Linus Torvalds
2019-04-15 5:19 ` Christoph Hellwig
2019-04-15 16:17 ` Linus Torvalds
2019-04-16 9:09 ` Martin Schwidefsky
2019-04-16 12:06 ` Martin Schwidefsky
2019-04-16 16:16 ` Linus Torvalds
2019-04-16 16:49 ` Linus Torvalds
2019-04-17 7:46 ` Martin Schwidefsky
2019-04-17 8:02 ` Martin Schwidefsky
2019-04-17 16:57 ` Linus Torvalds
2019-04-18 8:02 ` Martin Schwidefsky
2019-04-18 15:49 ` Linus Torvalds
2019-04-18 18:41 ` Martin Schwidefsky [this message]
2019-04-19 13:33 ` Martin Schwidefsky
2019-04-19 17:27 ` Linus Torvalds
2019-04-23 15:38 ` Martin Schwidefsky
2019-04-23 16:06 ` Linus Torvalds
2019-04-17 3:38 ` Michael Ellerman
2019-04-17 4:13 ` Linus Torvalds
2019-05-02 12:21 ` Greg KH
2019-05-02 14:17 ` Martin Schwidefsky
2019-05-02 14:31 ` Greg KH
2019-05-02 15:10 ` Martin Schwidefsky
2019-05-20 11:09 ` Greg KH
2019-05-03 13:31 ` Michael Ellerman
2019-05-02 23:15 ` Christoph Hellwig
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